Abstract:

An optical pickup includes: a first emitting unit to emit an optical beam
of a first wavelength; a second emitting unit to emit an optical beam of
a second wavelength; a third emitting unit to emit an optical beam of a
third wavelength; an object lens to condense optical beams emitted from
the first through third emitting units onto a signal recording face of an
optical disc; and a diffraction unit provided on one face of an optical
element or the object lens positioned on the optical path of the optical
beams of the first through third wavelengths; wherein the diffraction
unit includes a generally circular first diffraction region provided on
the innermost perimeter, a ring zone shaped second diffraction region
provided on the outer side of the first diffraction region, and a ring
zone shaped third diffraction region provided on the outer side of the
second diffraction region.

Claims:

1. An optical pickup comprising:a first emitting unit configured to emit
an optical beam of a first wavelength corresponding to a first optical
disc;a second emitting unit configured to emit an optical beam of a
second wavelength which is longer than said first wavelength,
corresponding to a second optical disc which is different from said first
optical disc;a third emitting unit configured to emit an optical beam of
a third wavelength which is longer than said second wavelength,
corresponding to a third optical disc which is different from said first
and second optical discs;an object lens configured to condense optical
beams emitted from said first through third emitting units onto a signal
recording face of an optical disc; anda diffraction unit provided on one
face of an optical element or said object lens positioned on the optical
path of said optical beams of said first through third
wavelengths;wherein said diffraction unit includesa generally circular
first diffraction region provided on the innermost perimeter,a ring zone
shaped second diffraction region provided on the outer side of said first
diffraction region, anda ring zone shaped third diffraction region
provided on the outer side of said second diffraction region;wherein said
first diffraction region has a first diffraction structure formed in a
ring zone shape and having a predetermined depth, which emits diffracted
light of an order of said optical beam of the first wavelength which
passes therethrough and is condensed on the signal recording face of said
first optical disc via said object lens, emits diffracted light of an
order of said optical beam of the second wavelength which passes
therethrough and is condensed on the signal recording face of said second
optical disc via said object lens, and emits diffracted light of an order
of said optical beam of the third wavelength which passes therethrough
and is condensed on the signal recording face of said third optical disc
via said object lens;and wherein said second diffraction region has a
second diffraction structure which is different from said first
diffraction structure formed in a ring zone shape and having a
predetermined depth, which emits diffracted light of an order of said
optical beam of the first wavelength which passes therethrough and is
condensed on the signal recording face of said first optical disc via
said object lens, emits diffracted light of an order of said optical beam
of the second wavelength which passes therethrough and is condensed on
the signal recording face of said second optical disc via said object
lens, and emits diffracted light such that diffracted light of an order
other than the order of said optical beam of the third wavelength which
passes therethrough and is condensed on the signal recording face of said
third optical disc via said object lens is dominant;and wherein said
third diffraction region has a third diffraction structure which is
different from said first and second diffraction structures formed in a
ring zone shape and having a predetermined depth, which emits diffracted
light of an order of said optical beam of the first wavelength which
passes therethrough and is condensed on the signal recording face of said
first optical disc via said object lens, emits diffracted light such that
diffracted light of an order other than the order of said optical beam of
the second wavelength which passes therethrough and is condensed on the
signal recording face of said second optical disc via said object lens is
dominant, and emits diffracted light such that diffracted light of an
order other than the order of said optical beam of the third wavelength
which passes therethrough and is condensed on the signal recording face
of said third optical disc via said object lens is dominant.

2. The optical pickup according to claim 1, wherein said first diffraction
regionemits diffracted light such that diffracted light of an order k1i
of said optical beam of the first wavelength which passes therethrough
and is condensed on the signal recording face of said first optical disc
via said object lens has maximum diffraction efficiency as to diffracted
light of another order,emits diffracted light such that diffracted light
of an order k2i of said optical beam of the second wavelength which
passes therethrough and is condensed on the signal recording face of said
second optical disc via said object lens has maximum diffraction
efficiency as to diffracted light of another order, andemits diffracted
light such that diffracted light of an order k3i of said optical beam of
the third wavelength which passes therethrough and is condensed on the
signal recording face of said third optical disc via said object lens has
maximum diffraction efficiency as to diffracted light of another
order;and wherein said first diffraction region emits diffracted light so
as to have the relation of k1i≧k2i>k3i wherein an order
diffracting toward the optical axis direction of the input optical beam
is a positive order.

4. The optical pickup according to claim 2, wherein said first diffraction
region has a staircase form diffraction structure formed in which a
staircase structure having a plurality of steps is continuously formed in
the radial direction of the ring zone;and wherein said second diffraction
region has a staircase form diffraction structure formed in which a
staircase form having a plurality of steps is continuously formed in the
radial direction of the ring zone, or a blazed form;and wherein said
third diffraction region has a diffraction structure formed of a blazed
form.

5. The optical pickup according to claim 2, wherein said first diffraction
region has a non-cyclical diffraction structure formed in which a
non-cyclical structure is formed in the radial direction of the ring
zone;and wherein said second diffraction region has a non-cyclical
diffraction structure formed in which a non-cyclical structure is formed
in the radial direction of the ring zone, or a blazed form;and wherein
said third diffraction region has a diffraction structure formed of a
blazed form.

7. The optical pickup according to claim 6, wherein said second
diffraction region has a staircase form diffraction structure formed in
which a staircase form having a plurality of steps is continuously formed
in the radial direction of the ring zone, or a non-cyclical diffraction
structure formed in which a non-cyclical structure is formed in the
radial direction of the ring zone;and wherein said second diffraction
regionemits diffracted light such that diffracted light of an order k1m
of said optical beam of the first wavelength which passes therethrough
and is condensed on the signal recording face of said first optical disc
via said object lens has maximum diffraction efficiency as to diffracted
light of another order, andemits diffracted light such that diffracted
light of an order k2m of said optical beam of the second wavelength which
passes therethrough and is condensed on the signal recording face of said
second optical disc via said object lens has maximum diffraction
efficiency as to diffracted light of another order;and wherein said k1m
and k2m are (+1, +1), (-1, -1), (0, +2), (0, -2), (0, +1), (0, -1), (+1,
0), (-1, 0), (+1, -1), or (-1, +1), respectively.

8. The optical pickup according to claim 6, wherein said second
diffraction region has a blazed form diffraction structure formed;and
wherein said second diffraction regionemits diffracted light such that
diffracted light of an order k1m of said optical beam of the first
wavelength which passes therethrough and is condensed on the signal
recording face of said first optical disc via said object lens has
maximum diffraction efficiency as to diffracted light of another order,
andemits diffracted light such that diffracted light of an order k2m of
said optical beam of the second wavelength which passes therethrough and
is condensed on the signal recording face of said second optical disc via
said object lens has maximum diffraction efficiency as to diffracted
light of another order;and wherein said k1m and k2m are (+3, +2), (-3,
-2), (+2, +1), (-2, -1), (+1, +1), or (-1, -1), respectively.

9. The optical pickup according to claim 2, wherein at the time of input
to the input side face of the closer-disposed element, of said object
lens or the optical element to which said diffraction unit has been
provided, to said first through third emitting units, the optical beam of
the first wavelength is input as generally parallel light, and the
optical beams of the second and third wavelengths as diffused light.

10. The optical pickup according to claim 1, wherein said first through
third diffraction regions emit diffracted light such that diffracted
light of said optical beams of the first through third wavelengths
passing therethrough of an order other than zero order is dominant.

11. The optical pickup according to claim 1, wherein said first through
third diffraction regions each have a staircase form diffraction
structure formed in which a staircase form having a plurality of steps is
continuously formed in the radial direction of the ring zone.

12. The optical pickup according to claim 1, wherein said first and second
diffraction regions each have a staircase form diffraction structure
formed in which a staircase form having a plurality of steps is
continuously formed in the radial direction of the ring zone;and wherein
said third diffraction region has a blazed form diffraction structure
formed.

13. The optical pickup according to claim 1, further comprising a
divergent angle converting element configured to convert the divergent
angle of the optical beams emitted from said first through third emitting
units;wherein said divergent angle converting element converts the
divergent angle of said optical beams of the first through third
wavelengths, and at the time of input to the input side face of the
closer-disposed element of said object lens or the optical element to
which said diffraction unit has been provided to said first through third
emitting units, the optical beams of the first and second wavelengths are
input as generally parallel light, and the optical beam of the third
wavelength as converged light or diffused light.

14. The optical pickup according to claim 1, wherein said first
diffraction regionemits diffracted light such that diffracted light of an
order k1i of said optical beam of the first wavelength which passes
therethrough and is condensed on the signal recording face of said first
optical disc via said object lens has maximum diffraction efficiency as
to diffracted light of another order,emits diffracted light such that
diffracted light of an order k2i of said optical beam of the second
wavelength which passes therethrough and is condensed on the signal
recording face of said second optical disc via said object lens has
maximum diffraction efficiency as to diffracted light of another order,
andemits diffracted light such that diffracted light of an order k3i of
said optical beam of the third wavelength which passes therethrough and
is condensed on the signal recording face of said third optical disc via
said object lens has maximum diffraction efficiency as to diffracted
light of another order;and wherein said k1i and k2i are of opposite
signs, and said k2i and k3i are of the same sign.

16. The optical pickup according to claim 1, wherein said first through
third diffraction regions are formed to a size such that said optical
beam of the first wavelength passing therethrough becomes a corresponding
first numerical aperture;and wherein said first and second diffraction
regions are formed to a size such that said optical beam of the second
wavelength passing therethrough becomes a corresponding second numerical
aperture;and wherein said first diffraction region is formed to a size
such that said optical beam of the third wavelength passing therethrough
becomes a corresponding third numerical aperture.

17. The optical pickup according to claim 1, wherein said first
diffraction regionemits diffracted light such that diffracted light of an
order k1i of said optical beam of the first wavelength which passes
therethrough and is condensed on the signal recording face of said first
optical disc via said object lens has maximum diffraction efficiency as
to diffracted light of another order,emits diffracted light such that
diffracted light of an order k2i of said optical beam of the second
wavelength which passes therethrough and is condensed on the signal
recording face of said second optical disc via said object lens has
maximum diffraction efficiency as to diffracted light of another order,
andemits diffracted light such that diffracted light of an order k3i of
said optical beam of the third wavelength which passes therethrough and
is condensed on the signal recording face of said third optical disc via
said object lens has maximum diffraction efficiency as to diffracted
light of another order;and wherein said first diffraction region emits
diffracted light such that said k1i is +1, said k2i is +1, and said k3i
is +1, wherein an order diffracting toward the optical axis direction of
the input optical beam is a positive order.

18. The optical pickup according to claim 17, further comprising a
divergent angle converting element configured to convert the divergent
angle of the optical beams emitted from said first through third emitting
units;wherein said divergent angle converting element converts the
divergent angle of said optical beams of the first through third
wavelengths, and at the time of input to the input side face of the
closer-disposed element of said object lens or the optical element to
which said diffraction unit has been provided to said first through third
emitting units, the optical beam of the first wavelength is input as
generally parallel light, and at least one of the optical beams of the
second and third wavelengths as diffused light.

19. The optical pickup according to claim 17, further comprising a
divergent angle converting element configured to convert the divergent
angle of the optical beams emitted from said first through third emitting
units;wherein said divergent angle converting element converts the
divergent angle of said optical beams of the first through third
wavelengths, and at the time of input to the input side face of the
closer-disposed element of said object lens or the optical element to
which said diffraction unit has been provided to said first through third
emitting units, the optical beam of the first wavelength is input as
generally parallel light, and the optical beams of the second and third
wavelengths as dispersed light.

20. The optical pickup according to claim 17, wherein said second
diffraction regionemits diffracted light such that diffracted light of an
order k1m of said optical beam of the first wavelength which passes
therethrough and is condensed on the signal recording face of said first
optical disc via said object lens has maximum diffraction efficiency as
to diffracted light of another order, andemits diffracted light such that
diffracted light of an order k2m of said optical beam of the second
wavelength which passes therethrough and is condensed on the signal
recording face of said second optical disc via said object lens has
maximum diffraction efficiency as to diffracted light of another
order;and wherein said k1m and k2m are (+1, +1) or (+3, +2),
respectively.

21. The optical pickup according to claim 17, wherein said first
diffraction region has a diffraction structure formed of a blazed
form;and wherein said second diffraction region has a staircase form
diffraction structure formed in which a staircase form having a plurality
of steps is continuously formed in the radial direction of the ring zone,
or a blazed form;and wherein said third diffraction region has a
staircase form diffraction structure formed in which a staircase form
having a plurality of steps is continuously formed in the radial
direction of the ring zone, or a blazed form.

22. The optical pickup according to claim 17, wherein said first through
third diffraction regions are formed to a size such that said optical
beam of the first wavelength passing therethrough becomes a corresponding
first numerical aperture;and wherein said first and second diffraction
regions are formed to a size such that said optical beam of the second
wavelength passing therethrough becomes a corresponding second numerical
aperture;and wherein said first diffraction region is formed to a size
such that said optical beam of the third wavelength passing therethrough
becomes a corresponding third numerical aperture.

23. An optical disc device comprising:driving means configured to hold and
rotationally drive an optical disc optionally selected from at leasta
first optical disc,a second optical disc of a different type from said
first optical disc, anda third optical disc of a different type from said
first and second optical discs; andan optical pickup configured to
selectively irradiate multiple optical beams of different wavelengths to
an optical disc rotationally driven by said driving means, so as to
record and/or play information signals;said optical pickup includinga
first emitting unit configured to emit an optical beam of a first
wavelength corresponding to said first optical disc,a second emitting
unit configured to emit an optical beam of a second wavelength which is
longer than said first wavelength, corresponding to said second optical
disc,a third emitting unit configured to emit an optical beam of a third
wavelength which is longer than said second wavelength, corresponding to
said third optical disc,an object lens configured to condense optical
beams emitted from said first through third emitting units onto a signal
recording face of an optical disc, anda diffraction unit provided on one
face of an optical element or said object lens, positioned on the optical
path of said optical beams of the first through third wavelengths;wherein
said diffraction unit includes a generally circular first diffraction
region provided on the innermost perimeter, a ring zone shaped second
diffraction region provided on the outer side of said first diffraction
region, and a ring zone shaped third diffraction region provided on the
outer side of said second diffraction region;wherein said first
diffraction region has a first diffraction structure formed in a ring
zone shape and having a predetermined depth, whichemits diffracted light
of an order of said optical beam of the first wavelength which passes
therethrough and is condensed on the signal recording face of said first
optical disc via said object lens,emits diffracted light of an order of
said optical beam of the second wavelength which passes therethrough and
is condensed on the signal recording face of said second optical disc via
said object lens, andemits diffracted light of an order of said optical
beam of the third wavelength which passes therethrough and is condensed
on the signal recording face of said third optical disc via said object
lens;and wherein said second diffraction region has a second diffraction
structure which is different from said first diffraction structure formed
in a ring zone shape and having a predetermined depth, which emits
diffracted light of an order of said optical beam of the first wavelength
which passes therethrough and is condensed on the signal recording face
of said first optical disc via said object lens, emits diffracted light
of an order of said optical beam of the second wavelength which passes
therethrough and is condensed on the signal recording face of said second
optical disc via said object lens, and emits diffracted light such that
diffracted light of an order other than the order of said optical beam of
the third wavelength which passes therethrough and is condensed on the
signal recording face of said third optical disc via said object lens is
dominant;and wherein said third diffraction region has a third
diffraction structure which is different from said first and second
diffraction structures formed in a ring zone shape and having a
predetermined depth, whichemits diffracted light of an order of said
optical beam of the first wavelength which passes therethrough and is
condensed on the signal recording face of said first optical disc via
said object lens,emits diffracted light such that diffracted light of an
order other than the order of said optical beam of the second wavelength
which passes therethrough and is condensed on the signal recording face
of said second optical disc via said object lens is dominant, andemits
diffracted light such that diffracted light of an order other than the
order of said optical beam of the third wavelength which passes
therethrough and is condensed on the signal recording face of said third
optical disc via said object lens is dominant.

24. An object lens used with an optical pickup configured to irradiate
optical beams on at leasta first optical disc,a second optical disc of a
different type from said first optical disc, anda third optical disc of a
different type from said first and second optical discs,so as to record
and/or play information signals, with said object lens condensingan
optical beam of a first wavelength corresponding to said first optical
disc,an optical beam of a second wavelength which is longer than said
first wavelength, corresponding to said second optical disc, andan
optical beam of a third wavelength which is longer than said second
wavelength, corresponding to said third optical disc, onto a signal
recording face of a corresponding optical disc, said object lens
comprising:a diffraction unit provided on the input side face or output
side face;wherein said diffraction unit includes a generally circular
first diffraction region provided on the innermost perimeter, a ring zone
shaped second diffraction region provided on the outer side of said first
diffraction region, and a ring zone shaped third diffraction region
provided on the outer side of said second diffraction region;wherein said
first diffraction region has a first diffraction structure formed in a
ring zone shape and having a predetermined depth, whichemits diffracted
light of an order of said optical beam of the first wavelength which
passes therethrough and is condensed on the signal recording face of said
first optical disc via said object lens, emitsdiffracted light of an
order of said optical beam of the second wavelength which passes
therethrough and is condensed on the signal recording face of said second
optical disc via said object lens, and emitsdiffracted light of an order
of said optical beam of the third wavelength which passes therethrough
and is condensed on the signal recording face of said third optical disc
via said object lens;and wherein said second diffraction region has a
second diffraction structure which is different from said first
diffraction structure formed in a ring zone shape and having a
predetermined depth, whichemits diffracted light of an order of said
optical beam of the first wavelength which passes therethrough and is
condensed on the signal recording face of said first optical disc via
said object lens,emits diffracted light of an order of said optical beam
of the second wavelength which passes therethrough and is condensed on
the signal recording face of said second optical disc via said object
lens, andemits diffracted light such that diffracted light of an order
other than the order of said optical beam of the third wavelength which
passes therethrough and is condensed on the signal recording face of said
third optical disc via said object lens is dominant;and wherein said
third diffraction region has a third diffraction structure which is
different from said first and second diffraction structures formed in a
ring zone shape and having a predetermined depth, whichemits diffracted
light of an order of said optical beam of the first wavelength which
passes therethrough and is condensed on the signal recording face of said
first optical disc via said object lens,emits diffracted light such that
diffracted light of an order other than the order of said optical beam of
the second wavelength which passes therethrough and is condensed on the
signal recording face of said second optical disc via said object lens is
dominant, andemits diffracted light such that diffracted light of an
order other than the order of said optical beam of the third wavelength
which passes therethrough and is condensed on the signal recording face
of said third optical disc via said object lens is dominant.

25. An optical disc device comprising:a driving unit configured to hold
and rotationally drive an optical disc optionally selected from at leasta
first optical disc,a second optical disc of a different type from said
first optical disc, anda third optical disc of a different type from said
first and second optical discs; andan optical pickup configured to
selectively irradiate multiple optical beams of different wavelengths to
an optical disc rotationally driven by said driving unit, so as to record
and/or play information signals;said optical pickup includinga first
emitting unit configured to emit an optical beam of a first wavelength
corresponding to said first optical disc,a second emitting unit
configured to emit an optical beam of a second wavelength which is longer
than said first wavelength, corresponding to said second optical disc,a
third emitting unit configured to emit an optical beam of a third
wavelength which is longer than said second wavelength, corresponding to
said third optical disc,an object lens configured to condense optical
beams emitted from said first through third emitting units onto a signal
recording face of an optical disc, anda diffraction unit provided on one
face of an optical element or said object lens, positioned on the optical
path of said optical beams of the first through third wavelengths;wherein
said diffraction unit includes a generally circular first diffraction
region provided on the innermost perimeter, a ring zone shaped second
diffraction region provided on the outer side of said first diffraction
region, and a ring zone shaped third diffraction region provided on the
outer side of said second diffraction region;wherein said first
diffraction region has a first diffraction structure formed in a ring
zone shape and having a predetermined depth, whichemits diffracted light
of an order of said optical beam of the first wavelength which passes
therethrough and is condensed on the signal recording face of said first
optical disc via said object lens,emits diffracted light of an order of
said optical beam of the second wavelength which passes therethrough and
is condensed on the signal recording face of said second optical disc via
said object lens, andemits diffracted light of an order of said optical
beam of the third wavelength which passes therethrough and is condensed
on the signal recording face of said third optical disc via said object
lens;and wherein said second diffraction region has a second diffraction
structure which is different from said first diffraction structure formed
in a ring zone shape and having a predetermined depth, whichemits
diffracted light of an order of said optical beam of the first wavelength
which passes therethrough and is condensed on the signal recording face
of said first optical disc via said object lens,emits diffracted light of
an order of said optical beam of the second wavelength which passes
therethrough and is condensed on the signal recording face of said second
optical disc via said object lens, andemits diffracted light such that
diffracted light of an order other than the order of said optical beam of
the third wavelength which passes therethrough and is condensed on the
signal recording face of said third optical disc via said object lens is
dominant;and wherein said third diffraction region has a third
diffraction structure which is different from said first and second
diffraction structures formed in a ring zone shape and having a
predetermined depth, whichemits diffracted light of an order of said
optical beam of the first wavelength which passes therethrough and is
condensed on the signal recording face of said first optical disc via
said object lens,emits diffracted light such that diffracted light of an
order other than the order of said optical beam of the second wavelength
which passes therethrough and is condensed on the signal recording face
of said second optical disc via said object lens is dominant, andemits
diffracted light such that diffracted light of an order other than the
order of said optical beam of the third wavelength which passes
therethrough and is condensed on the signal recording face of said third
optical disc via said object lens is dominant.

Description:

CROSS REFERENCES TO RELATED APPLICATIONS

[0001]The present invention contains subject matter related to Japanese
Patent Application JP 2007-197961 filed in the Japanese Patent Office on
Jul. 30, 2007, Japanese Patent Application JP 2008-063383 filed in the
Japanese Patent Office on Mar. 12, 2008, and Japanese Patent Application
JP 2007-303610 filed in the Japanese Patent Office on Nov. 22, 2007, the
entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to an object lens used with an optical
pickup for recording and/or playing information signals to and/or from
three different types of optical discs, the optical pickup, and an
optical disc device using the optical pickup.

[0004]2. Description of the Related Art

[0005]As of recent, there has been proposed, as a next-generation optical
disc format, an optical disc capable of high density recording, wherein
signals are recorded and played using an optical beam of blue-violet
semiconductor laser beam having a wavelength around 405 nm (hereafter
referred to as "high density recording optical disc"). This high density
recording optical disc is being proposed with a structure wherein the
cover layer for protecting the signal recording layer is thin, 0.1 mm for
example.

[0006]In providing an optical pickup compatible with such high density
recording optical discs, compatibility with CDs (Compact Discs) using a
wavelength around 785 nm and DVDs (Digital Versatile Discs) using a
wavelength around 655 nm, according to the related art, is desirable.
That is to say, there is demand for an optical pickup and optical disc
device having compatibility among optical discs of multiple formats with
different disc structures and accordingly different laser specifications.

[0007]There has been related art which realizes recoding or playing of
information signals to/from three types of optical discs of different
formats, such as that shown in FIG. 60, for example. This arrangement
involves having two types of object lenses and two types of optical
systems, one corresponding to DVD and CD, and the other to high density
recording optical discs, with the object lenses being switched over
according to the wavelength being used.

[0008]An optical pickup 430 shown in FIG. 60 realizes recording and/or
playing to and/or from optical discs of different types, by having two
types of object lenses, object lens 433 and object lens 434. The optical
pickup 430 has a light source unit 432, such as a laser diode or the
like, including an emitting unit for emitting an optical beam of a
wavelength around 785 nm for optical discs such as CDs and an emitting
unit for emitting an optical beam of a wavelength around 655 nm for
optical discs such as DVDs, a light source unit 431, such as a laser
diode or the like, including an emitting unit for emitting an optical
beam of a wavelength around 405 nm for high density recording optical
discs, an object lens 434 for optical discs such as DVDs and CDs, and an
object lens 433 for high density recording optical discs. The optical
pickup also has collimator lenses 442A and 442B, quarter-wave plates 443A
and 443B, redirecting mirrors 444A and 444B, beam splitters 437 and 438,
gratings 439 and 440, a photosensor 445, a multi-lens 446, and so forth.

[0009]An optical beam of a wavelength around 785 nm emitted from the light
source 432 is transmitted through the beam splitter 437 and beam splitter
438, and is input to the object lens 434. The object lens 434 condenses
the beam onto the signal recording face of the optical disc having a
protective layer (cover layer) 1.1 mm thick.

[0010]In the same way, the optical beam of a wavelength around 655 nm
emitted from the light source 432 is input to the object lens 434 via
exactly the same optical path, and is condensed onto the signal recording
face of the optical disc having a protective layer 0.6 mm thick. Return
light of a wavelength of 785 nm and of a wavelength of 655 nm reflected
off of the signal recording face of the optical disc passes through the
beam splitter 438, and is detected by the photosensor 445 having a
photodetector or the like.

[0011]An optical beam of a wavelength around 405 nm emitted from the light
source 431 is reflected at the beam splitter 437, and is input to the
object lens 433 via the beam splitter 438. The object lens 433 condenses
the beam onto the signal recording face of the optical disc having a
protective layer 0.1 mm thick. Return light of a wavelength of 405 nm
reflected off of the signal recording face of the optical disc is
detected at the photosensor 445 via the beam splitter 438.

[0012]Thus, the optical pickup shown in FIG. 60 realizes recording and/or
playing of three different types of optical discs by having two types of
object lenses, the object lens 434 for DVDs and CDs, and the object lens
433 for high density recording optical discs, thereby realizing
compatibility between multiple types of optical discs.

SUMMARY OF THE INVENTION

[0013]However, optical pickups according to the related art such as
described above have the following problems. First, each optical disc has
a different optimal inclination of object lens, and with the
above-described optical pickup, using two object lenses 433 and 434 means
that the attachment angle of the actuator of the object lenses 433 and
434 to lens holders may be unsuitable, resulting in a situation wherein
an optimal object lens inclination cannot be realized as to an optical
disc, resulting in deterioration in quality of played signals. Also, with
the above-described optical pickup, increase in the number of parts which
need to be placed along the optical path of each of the two optical
systems, such as redirecting mirrors, collimator lenses, quarter-wave
plates, and so on, is necessitated due to using the two object lenses 433
and 434, causing the problem of increased cost and increased size of the
optical pickup. Further, with the above-described optical pickup, there
is the need to mount the two object lenses 433 and 434 on an object lens
driving actuator, resulting in a heavier actuator, of which the
sensitivity is thus lowered.

[0014]As opposed to this arrangement, there is being studied an optical
pickup wherein the above problems are solved and further optical parts
are simplified, by having a single object lens used in common by the
multiple types of optical discs and the three types of wavelengths. A
basic principle for providing an object lens corresponding to optical
beams of the three types of wavelengths is to provide a diffraction unit
such as a diffraction optical element in the optical path upstream of the
object lens, thereby inputting the beam into the object lens in the state
of diffusion/convergent light, thereby correcting spherical aberration
occurring due to the combination of usage wavelength and the media.

[0015]However, with the optical pickup being studied according to the
related art, the structure has involved diffraction units being provided
on multiple faces, or the diffractive face having a spherical face shape
differing from the spherical face of the object lens, or there being a
need to provide a liquid crystal device having a complex configuration in
the optical path upstream of the object lens. However, each of these
configurations have the lens units, diffraction units, liquid crystal
devices, etc., individually formed and then later assembled, meaning that
a rather high level of precision is necessary for positioning these and
adhering multiple diffraction faces, leading to more and increasingly
troublesome and complicated steps in manufacturing, and problems of
failure to meet the necessary precision.

[0016]Also, for example, there has been proposed in Japanese Unexamined
Patent Application Publication No. 2004-265573 an optical pickup wherein
a diffraction unit is provided on the entire face, but this has only been
successful in realizing compatibility of two wavelengths. In order to
realize compatibility of three wavelengths, there is the need to
separately provide an object lens corresponding to the other wavelength,
and increase in the number of optical parts, and accordingly increased
complication of the arrangement, has been a problem.

[0017]There has been realized the need to provide an object lens and
condensing optical device used in an optical pickup realizing recording
and/or playing information signals by condensing optical beams on three
types of optical discs with different usage wavelengths, using a single
shared object lens, without a complicated configuration, the optical
pickup, and an optical disc device using the optical pickup.

[0018]An object lens, according to an embodiment of the present invention,
used with an optical pickup configured to irradiate optical beams on at
least a first optical disc, a second optical disc of a different type
from the first optical disc, and a third optical disc of a different type
from the first and second optical discs, so as to record and/or play
information signals, with the object lens condensing an optical beam of a
first wavelength corresponding to the first optical disc, an optical beam
of a second wavelength which is longer than the first wavelength,
corresponding to the second optical disc, and an optical beam of a third
wavelength which is longer than the second wavelength, corresponding to
the third optical disc, onto a signal recording face of a corresponding
optical disc, the object lens including: a diffraction unit provided on
the input side face or output side face; wherein the diffraction unit
includes a generally circular first diffraction region provided on the
innermost perimeter, a ring zone shaped second diffraction region
provided on the outer side of the first diffraction region, and a ring
zone shaped third diffraction region provided on the outer side of the
second diffraction region; wherein the first diffraction region has a
first diffraction structure formed in a ring zone shape and having a
predetermined depth, which emits diffracted light of an order of the
optical beam of the first wavelength which passes therethrough and is
condensed on the signal recording face of the first optical disc via the
object lens, emits diffracted light of an order of the optical beam of
the second wavelength which passes therethrough and is condensed on the
signal recording face of the second optical disc via the object lens, and
emits diffracted light of an order of the optical beam of the third
wavelength which passes therethrough and is condensed on the signal
recording face of the third optical disc via the object lens; and wherein
the second diffraction region has a second diffraction structure which is
different from the first diffraction structure formed in a ring zone
shape and having a predetermined depth, which emits diffracted light of
an order of the optical beam of the first wavelength which passes
therethrough and is condensed on the signal recording face of the first
optical disc via the object lens, emits diffracted light of an order of
the optical beam of the second wavelength which passes therethrough and
is condensed on the signal recording face of the second optical disc via
the object lens, and emits diffracted light such that diffracted light of
an order other than the order of the optical beam of the third wavelength
which passes therethrough and is condensed on the signal recording face
of the third optical disc via the object lens is dominant; and wherein
the third diffraction region has a third diffraction structure which is
different from the first and second diffraction structures formed in a
ring zone shape and having a predetermined depth, which emits diffracted
light of an order of the optical beam of the first wavelength which
passes therethrough and is condensed on the signal recording face of the
first optical disc via the object lens, emits diffracted light such that
diffracted light of an order other than the order of the optical beam of
the second wavelength which passes therethrough and is condensed on the
signal recording face of the second optical disc via the object lens is
dominant, and emits diffracted light such that diffracted light of an
order other than the order of the optical beam of the third wavelength
which passes therethrough and is condensed on the signal recording face
of the third optical disc via the object lens is dominant.

[0019]An optical pickup according to an embodiment of the present
invention includes: a first emitting unit configured to emit an optical
beam of a first wavelength corresponding to a first optical disc; a
second emitting unit configured to emit an optical beam of a second
wavelength which is longer than the first wavelength, corresponding to a
second optical disc which is different from the first optical disc; a
third emitting unit configured to emit an optical beam of a third
wavelength which is longer than the second wavelength, corresponding to a
third optical disc which is different from the first and second optical
discs; and an object lens configured to condense optical beams emitted
from the first through third emitting units onto a signal recording face
of an optical disc; and a diffraction unit provided on one face of an
optical element or the object lens positioned on the optical path of the
optical beams of the first through third wavelengths; wherein the
diffraction unit includes a generally circular first diffraction region
provided on the innermost perimeter, a ring zone shaped second
diffraction region provided on the outer side of the first diffraction
region, and a ring zone shaped third diffraction region provided on the
outer side of the second diffraction region; wherein the first
diffraction region has a first diffraction structure formed in a ring
zone shape and having a predetermined depth, which emits diffracted light
of an order of the optical beam of the first wavelength which passes
therethrough and is condensed on the signal recording face of the first
optical disc via the object lens, emits diffracted light of an order of
the optical beam of the second wavelength which passes therethrough and
is condensed on the signal recording face of the second optical disc via
the object lens, and emits diffracted light of an order of the optical
beam of the third wavelength which passes therethrough and is condensed
on the signal recording face of the third optical disc via the object
lens; and wherein the second diffraction region has a second diffraction
structure which is different from the first diffraction structure formed
in a ring zone shape and having a predetermined depth, which emits
diffracted light of an order of the optical beam of the first wavelength
which passes therethrough and is condensed on the signal recording face
of the first optical disc via the object lens, emits diffracted light of
an order of the optical beam of the second wavelength which passes
therethrough and is condensed on the signal recording face of the second
optical disc via the object lens, and emits diffracted light such that
diffracted light of an order other than the order of the optical beam of
the third wavelength which passes therethrough and is condensed on the
signal recording face of the third optical disc via the object lens is
dominant; and wherein the third diffraction region has a third
diffraction structure which is different from the first and second
diffraction structures formed in a ring zone shape and having a
predetermined depth, which emits diffracted light of an order of the
optical beam of the first wavelength which passes therethrough and is
condensed on the signal recording face of the first optical disc via the
object lens, emits diffracted light such that diffracted light of an
order other than the order of the optical beam of the second wavelength
which passes therethrough and is condensed on the signal recording face
of the second optical disc via the object lens is dominant, and emits
diffracted light such that diffracted light of an order other than the
order of the optical beam of the third wavelength which passes
therethrough and is condensed on the signal recording face of the third
optical disc via the object lens is dominant.

[0020]An optical disc device according to the present invention includes:
a driving unit configured to hold and rotationally drive an optical disc
optionally selected from at least a first optical disc, a second optical
disc of a different type from the first optical disc, and a third optical
disc of a different type from the first and second optical discs; and an
optical pickup configured to selectively irradiate multiple optical beams
of different wavelengths to an optical disc rotationally driven by the
driving unit, so as to record and/or play information signals the optical
pickup used with the optical disc device being such as described above.

[0021]According to the above configurations, due to a diffraction unit
provided on one face of an optical element disposed on an optical path
between an emitting unit emitting optical beams and the signal recording
face of an optical disc, optical beams corresponding to each of thee
types of optical discs having different usage wavelengths can be suitably
condensed on the signal recording faces thereof with a single shared
object lens, thereby realizing three-wavelength compatibility with a
common object lens, and realizing excellent recording and/or playing of
signals to and from each optical disc, without a complicated structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a block circuit diagram illustrating an optical device to
which the present invention has been applied;

[0023]FIG. 2 is an optical path diagram illustrating the optical system of
an optical pickup to which the present invention has been applied, as a
first embodiment;

[0024]FIGS. 3A and 3B are diagram for describing the functions of a
diffraction optical element and object lens configuring the optical
pickup shown in FIG. 2, wherein FIG. 3A is a diagram illustrating an
optical beam in a case of generating +1 order diffracted light of an
optical beam of a first wavelength as to a first optical disc for
example, FIG. 3B is a diagram illustrating an optical beam in a case of
generating -1 order diffracted light of an optical beam of a second
wavelength as to a second optical disc for example, and FIG. 3C is a
diagram illustrating an optical beam in a case of generating -2 order
diffracted light of an optical beam of a third wavelength as to a third
optical disc for example;

[0025]FIG. 4 is a diagram for describing a diffraction optical element
configuring the optical pickup shown in FIG. 2, showing a correlated plan
view and cross-sectional view of the diffraction optical element;

[0026]FIGS. 5A through 5C are diagrams for describing the configuration of
the diffraction unit provided on one face of the diffraction optical
element shown in FIG. 4, wherein FIG. 5A is a cross-sectional view
illustrating an example of a first diffraction region provided as an
inner ring zone of the diffraction unit, FIG. 5B is a cross-sectional
view illustrating an example of a second diffraction region provided as a
middle ring zone of the diffraction unit, and FIG. 5C is a
cross-sectional view illustrating an example of a third diffraction
region provided as an outer ring zone of the diffraction unit;

[0027]FIG. 6 is a cross-sectional view illustrating an example wherein a
blazed form diffraction structure has been formed, as another example of
the inner ring zone, middle ring zone, and outer ring zone, configuring
the diffraction unit;

[0028]FIGS. 7A through 7C show graphs for calculating the diffraction
efficiency of an inner ring zone configuration example 1 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=4, and (k1i, k2i, k3i)=(+1, -1, -2);

[0029]FIGS. 8A through 8C show graphs for calculating the diffraction
efficiency of an inner ring zone configuration example 2 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=6, and (k1i, k2i, k3i)=(+1, -2, -3);

[0030]FIGS. 9A through 9C show graphs for calculating the diffraction
efficiency of an inner ring zone configuration example 3 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=5, and (k1i, k2i, k3i)=(+2, -1, -2);

[0031]FIGS. 10A through 10C show graphs for calculating the diffraction
efficiency of an inner ring zone configuration example 4 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=6, and (k1i, k2i, k3i)=(+2, -2, -3);

[0032]FIGS. 11A through 11C show graphs for calculating the diffraction
efficiency of a middle ring zone configuration example 1 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=3, and (k1m, k2m, k3m)=(-1, +1, +2);

[0033]FIGS. 12A through 12C show shows graphs for calculating the
diffraction efficiency of a middle ring zone configuration example 2
according to the first embodiment, illustrating the change in the
diffraction efficiency of the optical beams of each wavelength as to
change in the groove depth d in a case wherein S=5, and (k1m, k2m,
k3m)=(-1, +2, +3);

[0034]FIGS. 13A through 13C show graphs for calculating the diffraction
efficiency of a middle ring zone configuration example 3 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=5, and (k1m, k2m, k3m)=(-2, +1, +2);

[0035]FIGS. 14A through 14C show shows graphs for calculating the
diffraction efficiency of an outer ring zone configuration example 1
according to the first embodiment, illustrating the change in the
diffraction efficiency of the optical beams of each wavelength as to
change in the groove depth d in a case wherein S=2, and (k1o, k2o,
k3o)=(-1, +1, +2);

[0036]FIGS. 15A through 15C show shows graphs for calculating the
diffraction efficiency of an outer ring zone configuration example 2
according to the first embodiment, illustrating the change in the
diffraction efficiency of the optical beams of each wavelength as to
change in the groove depth d in a case wherein S=5, and (k1o, k2o,
k3o)=(+1, -2, -3);

[0037]FIGS. 16A through 16C show graphs for calculating the diffraction
efficiency of an outer ring zone configuration example 3 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=5, and (k1o, k2o, k3o)=(+2, -1, -2);

[0038]FIGS. 17A through 17C show graphs for calculating the diffraction
efficiency of an outer ring zone configuration example 4 according to the
first embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=5, and (k1o, k2o, k3o)=(-2, +2, +3);

[0039]FIGS. 18A and 18B are diagrams describing an example of a condensing
optical device making up the optical pickup to which the present
invention has been applied, according to the first embodiment, wherein
FIG. 18A is a side view illustrating an example of a condensing optical
device configured of a diffraction optical element having a diffraction
unit on the incident side thereof and an object lens, and FIG. 18B is a
side view illustrating a diffraction optical device according to an
example wherein a diffraction unit is integrally formed on the incident
side face of an object lens;

[0040]FIG. 19 is an optical path diagram illustrating another example of
the optical system of an optical pickup to which the present invention
has been applied, as a first embodiment;

[0041]FIG. 20 is an optical path diagram illustrating the optical system
of an optical pickup to which the present invention has been applied, as
a second embodiment;

[0042]FIGS. 21A through 21C are diagrams for describing the functions of
the diffraction optical element and object lens configuring the optical
pickup shown in FIG. 20, wherein FIG. 21A is a diagram illustrating an
optical beam in a case of generating +1 order diffracted light of an
optical beam of a first wavelength as to a first optical disc for
example, FIG. 21B is a diagram illustrating an optical beam in a case of
generating +1 order diffracted light of an optical beam of a second
wavelength as to a second optical disc for example, and FIG. 21C is a
diagram illustrating an optical beam in a case of generating +1 order
diffracted light of an optical beam of a third wavelength as to a third
optical disc for example;

[0043]FIG. 22 is a diagram for describing the diffraction optical element
configuring the optical pickup shown in FIG. 20, showing a correlated a
plan view and cross-sectional view of the diffraction optical element;

[0044]FIGS. 23A through 23C are diagrams for describing the configuration
of the diffraction unit provided on one face of the diffraction optical
element shown in FIG. 22, wherein FIG. 23A is a cross-sectional view
illustrating an example wherein first through third diffraction regions
provided as the inner ring zone, middle ring zone, and outer ring zone,
of the diffraction unit, respectively, are formed in a blazed form, FIG.
23B is a cross-sectional view illustrating another example of the second
diffraction region provided as the middle ring zone of the diffraction
unit, with the second diffraction region formed in a staircase form as
another example, and FIG. 23C is a cross-sectional view illustrating
another example of the third diffraction region provided as the outer
ring zone of the diffraction unit, with the third diffraction region
formed in a staircase form as another example;

[0045]FIG. 24 is a diagram for describing spherical aberration correction
possibility at the diffraction region of the diffraction unit configuring
the optical pickup which is used for diffracting the three wavelengths
(inner ring zone), showing points plotted according to the relation
between wavelength×diffraction order and protective layer
thickness, and the design line of the object lens, in a case wherein
(k1i, k2i, k3i)=(+1, +1, +1);

[0046]FIGS. 25A through 25C are diagrams illustrating the longitudinal
aberration of effect term ΔWn due to refractive index fluctuation
of the composition material under change in temperature, the effect term
ΔWλ due to wavelength fluctuation, and the sum ΔW of
the effect terms ΔWn and ΔWλ, wherein FIG. 25A is a
diagram illustrating the longitudinal aberration of each, in a case of
selecting a negative diffraction order, FIG. 25B is a diagram
illustrating the longitudinal aberration of each, in a case of selecting
a positive diffraction order, and FIG. 25C is a diagram illustrating the
longitudinal aberration of each, in a case of selecting a positive
diffraction order and also selecting relatively high order diffraction
orders for the middle ring zone and outer ring zone;

[0047]FIGS. 26A and 26B are diagrams for describing the longitudinal
aberration illustrated in FIGS. 25A through 25C, wherein FIG. 26A is
diagram illustrating the state of longitudinal aberration with a lens
having no aberration, and FIG. 26B is a diagram illustrating a line LB
indicating the state of longitudinal aberration with a lens having
aberration;

[0048]FIGS. 27A through 27C show graphs for calculating the diffraction
efficiency of an example 1 and example 2 of an inner ring zone according
to the second embodiment, illustrating the change in the diffraction
efficiency of the optical beams of each wavelength as to change in the
groove depth d in a case wherein S=∞, and (k1i, k2i, k3i)=(+1, +1,
+1);

[0049]FIGS. 28A through 28C show graphs for calculating the diffraction
efficiency of an example 1 of a middle ring zone according to the second
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=3, and (k1m, k2m, k3m)=(+1, +1, +1);

[0050]FIGS. 29A through 29C show graphs for calculating the diffraction
efficiency of an example 1 of an outer ring zone according to the second
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=∞, and (k1o, k2o, k3o)=(+1, +2, +2);

[0051]FIG. 30 is a diagram for describing flaring at the example 1 of an
outer ring zone according to the second embodiment, showing points
plotted according to the relation between wavelength×diffraction
order and protective layer thickness, and the design line of the object
lens, in a case wherein (k1o, k2o, k3o)=(+1, +2, +2);

[0052]FIGS. 31A through 31C show graphs for calculating the diffraction
efficiency of an example 2 of a middle ring zone according to the second
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=∞, and (k1m, k2m, k3m)=(+3, +2, +2);

[0053]FIGS. 32A through 32C show for calculating the diffraction
efficiency of an example 2 of an outer ring zone according to the second
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=∞, and (k1o, k2o, k3o)=(+4, +3, +3);

[0054]FIG. 33 is a diagram for describing flaring at the example 2 of a
middle ring zone according to the second embodiment, showing points
plotted according to the relation between wavelength×diffraction
order and protective layer thickness, and the design line of the object
lens, in a case wherein (k1m, k2m, k3m)=(+3, +2, +2);

[0055]FIG. 34 is a diagram for describing flaring at the example 2 of an
outer ring zone according to the second embodiment, showing points
plotted according to the relation between wavelength×diffraction
order and protective layer thickness, and the design line of the object
lens, in a case wherein (k1o, k2o, k3o)=(+4, +3, +3);

[0056]FIGS. 35A and 35B are diagrams for describing an example of a
condensing optical device making up the optical pickup to which the
present invention has been applied, according to the second embodiment,
wherein FIG. 35A is a side view illustrating a condensing optical device
configured of a diffraction optical element having a diffraction unit on
the incident side thereof, and an object lens, and FIG. 35B is a side
view illustrating a condensing optical device according to an example
wherein a diffraction unit is integrally formed on the incident side face
of the object lens;

[0057]FIG. 36 is an optical path diagram illustrating another example of
the optical system of an optical pickup to which the present invention
has been applied, as a second embodiment;

[0058]FIG. 37 is an optical path diagram illustrating the optical system
of an optical pickup to which the present invention has been applied, as
a third embodiment;

[0059]FIGS. 38A through 38C are diagrams for describing the functions of
the diffraction unit configuring the optical pickup shown in FIG. 37, and
is a diagram for describing the functions of a diffraction optical
element provided with a diffraction unit and having diffraction functions
and an object lens having refractive functions, with reference to an
example wherein the diffraction unit is provided to an optical element
separate from the object lens, wherein FIG. 38A is a diagram illustrating
an optical beam in a case of generating +1 order diffracted light of an
optical beam of a first wavelength as to a first optical disc for
example, FIG. 38B is a diagram illustrating an optical beam in a case of
generating -1 order diffracted light of an optical beam of a second
wavelength as to a second optical disc for example, and FIG. 38C is a
diagram illustrating an optical beam in a case of generating -2 order
diffracted light of an optical beam of a third wavelength as to a third
optical disc for example;

[0060]FIG. 39 is a diagram for describing the object lens configuring the
optical pickup shown in FIG. 37, showing a correlated plan view and
cross-sectional view of the object lens;

[0061]FIGS. 40A through 40C are diagrams for describing the configuration
of the diffraction unit provided on one face of the object lens shown in
FIG. 39, wherein FIG. 40A is a cross-sectional view illustrating a shape
as to the reference face as an example of the first diffraction region
provided as the inner ring zone of the diffraction unit, FIG. 40B is a
cross-sectional view illustrating a shape as to the reference face as an
example of the second diffraction region provided as the middle ring zone
of the diffraction unit, and FIG. 40C is a cross-sectional view
illustrating a shape as to the reference face as an example of the third
diffraction region provided as the outer ring zone of the diffraction
unit;

[0062]FIG. 41 is a diagram for describing spherical aberration correction
possibility at the diffraction region of the diffraction unit configuring
the optical pickup which is used for diffracting the three wavelengths
(inner ring zone) with reference to the inner ring zone of an example 1,
showing points plotted according to the relation between
wavelength×diffraction order and protective layer thickness, and
the design line of the object lens, in a case wherein (k1i, k2i,
k3i)=(+1, -1, -2);

[0063]FIG. 42 is a diagram conceptually illustrating that spherical
aberration can be corrected using divergent light, illustrating that the
plotted points Pλ1, Pλ2', and Pλ3' are positioned on a
straight line by the plot positions being shifted due to the second and
third wavelengths having been input in a state of divergent rays as
compared to the state in FIG. 41;

[0064]FIG. 43 is a diagram for describing the relation between the
diffraction orders k1 and k3 selected at the diffraction unit regarding
the first and third wavelengths, and the focal distance of the object
lens as to the third wavelength, and is a diagram illustrating the change
in the focal distance as to the third wavelength as the diffraction order
k3 of the third wavelength changes, for each diffraction order k1 of the
first wavelength;

[0065]FIGS. 44A through 44C show graphs for calculating the diffraction
efficiency of an example 1 of an inner ring zone according to the third
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=4, and (k1i, k2i, k3i)=(+1, -1, -2);

[0066]FIGS. 45A through 45C show graphs illustrating change in the
diffraction efficiency of a reference example for comparison with the
inner ring zone of the example 1 shown in FIG. 44, illustrating the
change in the diffraction efficiency of the optical beams of each
wavelength as to change in the groove depth d in a case of a blazed form
(S=∞), and (k1i, k2i, k3i)=(+1, +1, +1);

[0067]FIGS. 46A through 46C are diagrams for describing a technique for
determining the pitch of the diffraction structure, wherein FIG. 46A is a
diagram indicating the design phase amount φ to be provided to the
design wavelength λ0 at each position in the radial direction, FIG.
46B is a diagram illustrating indicating the phase amount φ' to be
actually provided at each position in the radial direction based on φ
in FIG. 46A, and FIG. 46C is a diagram conceptually illustrating the
shape of the diffraction structure for providing the phase amount φ'
shown in FIG. 46B;

[0068]FIG. 47 is a diagram illustrating another example of the middle ring
zone configuring the diffraction unit, and is a cross-sectional view
illustrating a shape as to the reference face as an example of the second
diffraction region where a staircase from diffraction structure is
formed;

[0069]FIG. 48 is a diagram for describing flaring at the middle ring zone
in the example 1 of the third embodiment, showing points plotted
according to the relation between wavelength×diffraction order and
protective layer thickness, and the design line of the object lens, in a
case wherein (k1m, k2m, k3m)=(+3, +2, +2);

[0070]FIG. 49 is a diagram for describing flaring at the outer ring zone
in the example 1 of the third embodiment, showing points plotted
according to the relation between wavelength×diffraction order and
protective layer thickness, and the design line of the object lens, in a
case wherein (k1o, k2o, k3o)=(+4, +2, +2);

[0071]FIGS. 50A through 50C show graphs for calculating the diffraction
efficiency of the example 1 of the middle ring zone according to the
third embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=∞, and (k1m, k2m, k3m)=(+3, +2, +2);

[0072]FIGS. 51A through 51C show graphs for calculating the diffraction
efficiency of the example 1 of the outer ring zone according to the third
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=∞, and (k1o, k2o, k3o)=(+4, +2, +2);

[0073]FIGS. 52A through 52C show graphs for calculating the diffraction
efficiency of the example 2 of the inner ring zone according to the third
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=3, and (k1i, k2i, k3i)=(0, -1, -2);

[0074]FIGS. 53A through 53C show graphs for calculating the diffraction
efficiency of the example 2 of the middle ring zone according to the
third embodiment, illustrating the change in the diffraction efficiency
of the optical beams of each wavelength as to change in the groove depth
d in a case wherein S=∞, and (k1m, k2m, k3m)=(0, -1, -3);

[0075]FIGS. 54A through 54C show graphs for calculating the diffraction
efficiency of the example 2 of the outer ring zone according to the third
embodiment, illustrating the change in the diffraction efficiency of the
optical beams of each wavelength as to change in the groove depth d in a
case wherein S=∞, and (k1o, k2o, k3o)=(+1, +1, +1);

[0076]FIG. 55 is a diagram for describing spherical aberration correction
possibility at the inner ring zone in the example 2 of the third
embodiment, showing points plotted according to the relation between
wavelength×diffraction order and protective layer thickness, and
the design line of the object lens, in a case wherein (k1i, k2i,
k3i)=(+0, -1, -2);

[0077]FIG. 56 is a diagram for describing flaring at the middle ring zone
in the example 2 of the third embodiment, showing points plotted
according to the relation between wavelength×diffraction order and
protective layer thickness, and the design line of the object lens, in a
case wherein (k1m, k2m, k3m)=(0, -1, -3);

[0078]FIG. 57 is a diagram for describing flaring at the outer ring zone
in the example 2 of the third embodiment, showing points plotted
according to the relation between wavelength×diffraction order and
protective layer thickness, and the design line of the object lens, in a
case wherein (k1o, k2o, k3o)=(+1, +1, +1);

[0079]FIGS. 58A and 58B are diagrams for describing an example of a
condensing optical device making up the optical pickup to which the
present invention has been applied, according to the third embodiment,
wherein FIG. 58A is a side view illustrating a condensing optical device
having a diffraction unit according to an example of being configured of
an object lens with a diffraction unit integrally formed on the incident
side thereof, and FIG. 58B is a side view illustrating a condensing
optical device according to an example configured of a diffraction
optical element having a diffraction unit formed on the incident side
thereof, and an object lens;

[0080]FIG. 59 is an optical path diagram illustrating another example of
the optical system of an optical pickup to which the present invention
has been applied; and

[0081]FIG. 60 is an optical path diagram illustrating an example of an
optical system of an optical pickup according to the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0082]An embodiment of an optical disc device using an optical pickup to
which the present invention has been applied will be described with
reference to the drawings.

<1> Overall Configuration of Optical Disc Device (FIG. 1)

[0083]An optical disc device 1 to which the present invention has been
applied includes, as shown in FIG. 1, an optical pickup 3 for performing
information recording/playing to and from an optical disc 2, a spindle
motor 4 serving as a driving device for rotationally driving the optical
disc 2, and a sled motor 5 for moving the optical pickup 3 in the radial
direction of the optical disc 2. The optical disc device 1 is an optical
disc device realizing compatibility between three standards, whereby
information can be recorded and/or played to/from three types of optical
discs with different formats, and optical discs with layered recording
layers. Note that the optical pickup in the optical disc device 1 is not
restricted to the optical pickup 3, and that later-described optical
pickups 103, 203, and so forth, may be used as well.

[0084]Optical discs used here include, for example, optical discs using
semiconductor laser of an emission wavelength around 785 nm, such as CD
(Compact Disc), CD-R (Recordable), CD-RW (ReWritable), and so forth,
optical discs using semiconductor laser of an emission wavelength around
655 nm, such as DVD (Digital Versatile Disc), DVD-R (Recordable), DVD-RW
(ReWritable), DVD+RW (ReWritable), and so forth, and further high density
recording optical discs using a semiconductor laser of a shorter emission
wavelength around 405 nm (blue-violet), capable of high density
recording, such as BD (Blu-ray Disc (a registered trademark)) and so
forth.

[0085]Hereinafter, the three types of optical discs 2 which the optical
disc device 1 records information to or plays information from will be
described as a first optical disc 11 such as BD, described above as being
capable of high density recording, which has a protective layer formed to
a thickness of around 0.1 mm and uses an optical beam of a wavelength
around 405 nm as the recording/playing beam, a second optical disc 12
such as DVD which has a protective layer formed to a thickness of around
0.6 mm and uses an optical beam of a wavelength around 655 nm as the
recording/playing beam, and a third optical disc 13 such as CD which has
a protective layer formed to a thickness of around 1.1 mm and uses an
optical beam of a wavelength around 785 nm as the recording/playing beam.

[0086]Driving of the spindle motor 4 and sled motor 5 of the optical disc
device 1 is controlled by a servo control unit 9 controlled based on
instructions from a system controller 7 also serving as a disc type
determination unit, depending on the type of disc, and are driven at a
certain revolution according to the first optical disc 11, second optical
disc 12, and third optical disc 13, for example.

[0087]The optical pickup 3 is an optical pickup having a three wavelength
compatible optical system, wherein optical beams of different wavelengths
are irradiated onto the recording layers of optical discs of different
standards from the protective layer side, and reflected light of the
optical beams off of the recording layer is detected. The optical pickup
3 outputs signals corresponding to each of the optical beams, from the
detected reflected light.

[0088]The optical disc device 1 includes a preamp 14 for generating focus
error signals, tracking error signals, RF signals, and so forth, based on
signals output from the optical pickup 3, a signal modulator/demodulator
and error correction code block (hereinafter referred to as signal
modulator/demodulator & ECC block) 15 for demodulating signals from the
preamp 14 or modulating signals from an external computer 17 or the like,
an interface 16, a D/A-A/D converter 18, and audio-visual processing unit
19, and an audio-visual signal input/output unit 20.

[0089]Based on the output form the photosensor, the preamp 14 generates
focus error signals by the astigmatic method or the like, generates
tracking error signals by the three-beam method, DPD, DPP, or the like,
further generates RF signals, and outputs the RF signals to the signal
modulator/demodulator & ECC block 15. Also, the preamp 14 outputs focus
error signals and tracking error signals to the servo control unit 9.

[0090]At the time of recording data to the first optical disc 11, the
signal modulator/demodulator & ECC block 15 performs error correction
processing according to LDC-ECC and BIS or the like on the digital
signals input from the interface 16 or D/A-A/D converter 18, and then
performs modulation such as 1-7PP or the like. At the time of recording
data to the second optical disc 12, the signal modulator/demodulator &
ECC block 15 performs error correction processing such as PC (Product
Code) or the like, and then performs modulation such as 8-16 modulation
or the like. At the time of recording data to the third optical disc 13,
the signal modulator/demodulator & ECC block 15 performs error correction
processing such as CIRC or the like, and then performs modulation such as
8-14 modulation or the like. The signal modulator/demodulator & ECC block
15 then outputs the modulated data to a laser control unit 21. Further,
when playing each of the optical discs, the signal modulator/demodulator
& ECC block 15 performs demodulation processing based on the RF signals
input from the preamp 14, and then further performs error correction
processing, and outputs the data to the interface 16 or D/A-A/D converter
18.

[0091]For an arrangement wherein data is to be compressed and recorded, a
compression/decompression unit may be provided between the signal
modulator/demodulator & ECC block 15 and the interface 16 or D/A-A/D
converter 18. In this case, the data is compressed with a format such as
MPEG2, MPEG4, or the like.

[0092]The servo control unit 9 receives input of focus error signals and
tracking error signals from the preamp 14. The servo control unit 9
generates focus servo signals and tracking servo signals such that the
focus error signals and tracking error signals become zero, and drive and
control an object lens driving unit such as a biaxial actuator or the
like driving the object lens, based on the servo signals. Also,
synchronization signals or the like are detected from the output from the
preamp 14, and servo control of the spindle motor is performed by CLV
(Constant Linear Velocity), CAV (Constant Angular Velocity), a
combination thereof, or the like.

[0093]The laser control unit 21 controls the laser source of the optical
pickup 3. Particularly, with this specific example, control is effected
by the laser control unit 21 such that the laser source output power
differs between the recording mode and playback mode. Further, control is
effected by the laser control unit 21 such that the laser source output
power differs depending on the type of the optical disc 2. The laser
control unit 21 switches over the laser source of the optical pickup 3
depending on the type of optical disc 2 detected by a disc type
determination unit 22.

[0094]The disc type determination unit 22 can detect the different formats
of the optical disc 2 by detecting change in the amount of reflected
light from the first through third optical discs 11, 12, and 13, from
difference in surface reflectivity, shape and other external differences,
and so forth.

[0095]Each block making up the optical disc device 1 is configured so as
to be capable of signal processing in accordance with the specifications
of the optical disc 2 which has been mounted, based on the detection
results at the disc type determination unit 22.

[0096]The system controller 7 controls the entire device in accordance
with the type of optical disc 2 determined at the disc type determination
unit 22. Also, the system controller 7 identifies the recording position
or playing position of the optical disc regarding which recording/playing
is to be performed, based on address information and TOC (Table of
Contents) information recorded in premastered bits or grooves or the like
on the innermost portion of the optical disc, and controls the components
based on the determined position, in accordance with operation input from
the user.

[0097]With the optical disc device 1 configured thus, the optical disc 2
is rotationally driven by the spindle motor 4, the sled motor 5 is driven
and controlled in accordance with control signals from the servo control
unit 9, and the optical pickup 3 is moved to a position corresponding to
the desired recording track on the optical disc 2, thereby performing
recording/playing of information to/from the optical disc 2.

[0098]Specifically, at the time of performing recording/playing with the
optical disc device 1, the servo control unit 9 rotates the optical disc
2 by CAV or CLV or a combination thereof. The optical pickup 3 irradiates
an optical beam from the light source onto the optical disc 2 and detects
the returning optical beam therefrom with the photosensor, generates
focus error signals and tracking error signals, and performs focus servo
and tracking servo control by driving the object lens with an object lens
driving mechanism, based on the focus error signals and tracking error
signals.

[0099]Also, at the time of recording with the optical disc device 1,
signals from an external computer 17 are input to the signal
modulator/demodulator & ECC block 15 via the interface 16. The signal
modulator/demodulator & ECC block 15 adds the above-described
predetermined error correction code to the digital data input from the
interface 16 or the D/A-A/D converter 18, and after performing further
predetermined modulation processing, generates recording signals. The
laser control unit 21 controls the laser light source of the optical
pickup 3 based on the recording signals generated at the signal
modulator/demodulator & ECC block 15, and records onto a predetermined
optical disc.

[0100]Also, at the time of playing information recorded in an optical disc
2 with the optical disc device 1, the signal modulator/demodulator & ECC
block 15 performs demodulation processing on signals detected with the
photosensor. In the event that the recorded signals demodulated by the
signal modulator/demodulator & ECC block 15 are for computer data
storage, these are output to the external computer 17 via the interface
16. Accordingly, the external computer 17 can operate based on the
signals recorded on the optical disc 2. Also, in the event that the
recorded signals demodulated by the signal modulator/demodulator & ECC
block 15 are for audio-visual, the signals are subjected to
digital/analog conversion at the D/A-A/D converter 18, and supplied to
the audio-visual processing unit 19. Audio-visual processing is performed
at the audio-visual processing unit 19, and signals are output to unshown
external speakers or a monitor, via the audio-visual signal input/output
unit 20.

[0101]Now, the recording/playing optical pickups 3, 103, 203, etc., used
with the above-described optical disc device 1, will be described in
detail.

<2> First Embodiment of Optical Pickup (FIGS. 2 through 19)

[0102]First, an optical pickup 3 to which the present invention is applied
will be described as a first embodiment of the optical pickup according
to the present invention, with reference to FIGS. 2 through 19. As
described above, the optical pickup 3 is an optical pickup which
selectively irradiates multiple optical beams with different wavelengths
onto three types of optical discs arbitrarily selected from first through
third optical discs 11, 12, and 13, of which the format such as the
thickness of the protective layer differs, thereby performing recording
and/or playing of information signals.

[0103]As shown in FIG. 2, the optical pickup 3 to which the present
invention has been applied includes a first light source 31 having a
first emitting unit for emitting an optical beam of a first wavelength, a
second light source 32 having a second emitting unit for emitting an
optical beam of a second wavelength longer than the first wavelength, a
third light source 33 having a third emitting unit for emitting an
optical beam of a third wavelength longer than the second wavelength, an
object lens 34 for condensing optical beams emitted from the emitting
unit of the first through third emitting units onto the signal recording
face of an optical disc 2, and a diffraction optical element 35 provided
on the optical path between the first through third emitting units and
the object lens 34.

[0104]Also, the optical pickup 3 includes a first beam splitter 36
provided between the second and third emitting units and the diffraction
optical element 35, serving as an optical path synthesizing unit for
synthesizing the optical paths of the optical beam of the second
wavelength that has been emitted from the second emitting unit and the
optical beam of the third wavelength that has been emitted from the third
emitting unit, a second beam splitter 37 provided between the first beam
splitter 36 and the diffraction optical element 35, serving as an optical
path synthesizing unit for synthesizing the optical path of the optical
beams of the second and third wavelengths of which the optical paths have
been synthesized by the first beam splitter 36 and the optical beam of
the first wavelength that has been emitted from the first emitting unit,
and a third beam splitter 38 provided between the second beam splitter 37
and the diffraction optical element 35, serving as an optical path
splitting unit for splitting the outgoing optical path of the optical
beams of the first through third wavelengths synthesized at the second
beam splitter 37 from the returning optical path of the optical beam of
the first through third wavelengths reflected off of the optical disc
(hereinafter also referred to as "return path").

[0105]Further, the optical pickup 3 has a first grating 39 provided
between the first emitting unit of the first light source unit 31 and the
second beam splitter 37, for diffracting the optical beam of the first
wavelength that has been emitted from the first emitting unit into three
beams, for detection of tracking error signals and so forth, a second
grating 40 provided between the second emitting unit of the second light
source unit 32 and the first beam splitter 36, for diffracting the
optical beam of the second wavelength that has been emitted from the
second emitting unit into three beams, for detection of tracking error
signals and so forth, and a third grating 41 provided between the third
emitting unit of the third light source unit 33 and the first beam
splitter 36, for diffracting the optical beam of the third wavelength
that has been emitted from the third emitting unit into three beams, for
detection of tracking error signals and so forth.

[0106]Also, the optical pickup 3 has a collimator lens 42 provided between
the third beam splitter 38 and the diffraction optical element 35,
serving as a divergent angle conversion unit for converting the divergent
angle of the optical beams of the first through third wavelengths of
which the optical paths have been synthesized at the third beam splitter
38 so as to be adjusted into a state of generally parallel light or a
state diffused or converged as to generally parallel light, and
outputting, a quarter-wave plate 43 provided between the collimator lens
42 and the diffraction optical element 35, so as to provide quarter-wave
phase difference to the optical beams of the first through third
wavelengths of which the divergent angle has been adjusted by the
collimator lens 42, and a redirecting mirror 44 provided between the
diffraction optical element 35 and the quarter-wave plate 43, for
redirecting by reflection the optical beam which has passed through the
above-described optical parts within a plane generally orthogonal to the
optical axis of the object lens 34 and diffraction optical element 35, so
as to emit the optical beam in the direction toward the optical axis of
the object lens 34 and diffraction optical element 35.

[0107]Further, the optical pickup 3 includes a photosensor 45 for
receiving and detecting the optical beams of the first through third
wavelengths split at the third beam splitter 38 on the return path from
the optical beam of the first through third wavelengths on the outgoing
path, and a multi lens 46 provided between the third beam splitter 38 and
the photosensor 45, for condensing optical beams of the first through
third wavelengths on the return path split at the third beam splitter 38
onto the photoreception face of a photodetector or the like of the
photosensor 45, and also providing astigmatism for detecting focus error
signals or the like.

[0108]The first light source 31 has a first emitting unit for emitting an
optical beam of a first wavelength around 405 nm onto the first optical
disc 11. The second light source 32 has a second emitting unit for
emitting an optical beam of a second wavelength around 655 nm onto the
second optical disc 12. The third light source 33 has a third emitting
unit for emitting an optical beam of a third wavelength around 785 nm
onto the third optical disc 13. Note that while the first through third
emitting units are configured disposed at individual light sources 31,
32, and 33, the invention is not restricted to this, and an arrangement
may be made wherein two emitting units of the first through third
emitting units are disposed at one light source and the remaining
emitting unit is disposed at another light source, or wherein the first
through third emitting units are disposed so as to form a light source at
generally the same position.

[0109]The object lens 34 condenses the input optical beams of the first
through third wavelengths onto the signal recording face of the optical
disc 2. The object lens 34 is movably held by an object lens driving
mechanism such as an unshown biaxial actuator or the like. The object
lens 34 is driven along two axes, one in the direction toward/away from
the optical disc 2, and the other in the radial direction of the optical
disc 2, by being moved by a biaxial actuator or the like based on the
tracking error signals and focus error signals generated from the RF
signals of the return light from the optical disc 2 that has been
detected at the photosensor 45. The object lens 34 condenses optical
beams emitted from the first through third emitting units such that the
optical beams are always focused on the signal recording face of optical
disc 2, and also causes the focused optical beam to track a recording
track formed on the signal recording face of the optical disc 2. Note
that a configuration wherein the later-described diffraction optical
element 35 is held by a lens holder of the object lens driving mechanism
where the object lens 34 is held so as to be integral with the object
lens 34 enables the later-described advantages of a diffraction unit 50
provided to the diffraction optical element 35 to be suitably manifested
at the time of field shift of the object lens 34 such as movement in the
tracking direction.

[0110]The diffraction optical element 35 has, as one face thereof for
example, a diffraction unit 50 having multiple diffraction regions on the
incident side face thereof, with the diffraction unit 50 diffracting each
of the optical beams of the first through third wavelengths passing
through each of the multiple diffraction regions into predetermined
orders and inputting into the object lens 34, i.e., inputting into the
object lens 34 as optical beams in a diffused state or converged state
having a predetermined divergent angle, whereby the single object lens 34
can be used to perform suitable condensing of the optical beams of the
first through third wavelengths such that spherical aberration does not
occur at the signal recording face of the three types of optical discs
corresponding to the optical beams of the first through third
wavelengths. The diffraction optical element 35 serves as a condensation
optical device along with the object lens 34 to appropriately perform
condensation such that no spherical aberration occurs at the signal
recording face of the three types of optical discs corresponding to the
optical beams of the three different wavelengths.

[0111]The diffraction optical element 35 having the diffraction unit 50
performs diffraction of the first wavelength optical beam BB0 which has
transmitted the diffraction unit 50 so as to become +1st order diffracted
beam BB1 and inputs to the object lens 34, i.e., as an optical beam in a
diffused state having a predetermined divergent angle, thereby
appropriately condensing on the signal recording face of the first
optical disc 11, as shown in FIG. 3A, performs diffraction of the second
wavelength optical beam BD0 which has transmitted the diffraction unit 50
so as to become -1st order diffracted beam BD1 and inputs to the object
lens 34, i.e., as an optical beam in a converged state having a
predetermined divergent angle, thereby appropriately condensing on the
signal recording face of the second optical disc 12, as shown in FIG. 3B,
and performs diffraction of the third wavelength optical beam BC0 which
has transmitted the diffraction unit 50 so as to become -2nd order
diffracted beam BC1 and inputs to the object lens 34, i.e., as a beam in
a converged state having a predetermined divergent angle, thereby
appropriately condensing on the signal recording face of the third
optical disc 13, as shown in FIG. 3C, for example, whereby suitable
condensation can be performed such that no spherical aberration occurs at
the signal recording face of the three types of optical discs, with a
single object lens 34. While description has been made here with an
example wherein optical beams of the same wavelength are made to be
diffracted beams of the same diffraction order at the multiple
diffraction regions of the diffraction unit 50, with reference to FIGS.
3A through 3C, the diffraction unit 50 configuring the optical pickup 3
to which the present invention is applied enables diffraction order
corresponding to each wavelength to be set for each region as described
later, so as to further reduce spherical aberration.

[0112]Specifically, as shown in FIGS. 4A and 4B, the diffraction unit 50
provided at the incident side face of the diffraction optical element 35
has a generally-circular first diffraction region 51 provided on the
innermost portion (hereinafter also referred to as "inner ring zone"), a
ring-shaped second diffraction region 52 provided on the outer side of
the first diffraction region 51 (hereinafter also referred to as "middle
ring zone"), and a ring-shaped third diffraction region 53 provided on
the outer side of the second diffraction region 52 (hereinafter also
referred to as "outer ring zone").

[0113]The first diffraction region 51 which is an inner ring zone has a
first diffraction structure formed having a ring shape with a
predetermined depth, and diffracts the optical beam of the first
wavelength that is transmitted therethrough such that diffracted light of
an order which condenses light so as to form an appropriate spot on the
signal recording face of the first optical disc via the object lens 34 is
dominant, i.e., such that maximum diffraction efficiency is manifested
regarding diffracted light of other orders.

[0114]The first diffraction region 51 diffracts the optical beam of the
second wavelength that is transmitted therethrough such that diffracted
light of an order which condenses light so as to form an appropriate spot
on the signal recording face of the second optical disc via the object
lens 34 is dominant, i.e., such that maximum diffraction efficiency is
manifested regarding diffracted light of other orders, by way of the
first diffraction structure.

[0115]The first diffraction region 51 diffracts the optical beam of the
third wavelength that is transmitted therethrough such that diffracted
light of an order which condenses light so as to form an appropriate spot
on the signal recording face of the third optical disc via the object
lens 34 is dominant, i.e., such that maximum diffraction efficiency is
manifested regarding diffracted light of other orders, by way of the
first diffraction structure.

[0116]Thus, the first diffraction region 51 has a diffraction structure
formed whereby diffracted light of a predetermined order is dominant in
the optical beam of each wavelength, thereby enabling correction and
reduction of spherical aberration at the time of optical beams of each
wavelength that have passed through the first diffraction region 51 and
become diffracted light of a predetermined order being condensed on the
signal recording face of the respective optical discs by the object lens
34.

[0117]Specifically, as shown in FIGS. 4 and 5A, the first diffraction
region 51 is formed with the cross-sectional form of ring shapes centered
on the optical axis being formed in a staircase-like form having a
predetermined depth (hereinafter also referred to as "groove depth") d
and a predetermined number of steps S (where S is a positive integer),
continuing in the radial direction (also referred to as a "multi-step
staircase form"). Note that the cross-sectional form of the ring shapes
in this diffraction structure means the cross-sectional form of the rings
taken along a plane including the radial direction of the rings, i.e., a
plane orthogonal to the tangential direction of the rings. Also, the
diffraction structure having the staircase form with a predetermined
number of steps S is a structure in which a staircase form having first
through S steps, each of which have generally the same depth, continuing
in the radial direction, which can be rephrased as saying that the
structure has first through S+1'th diffraction faces formed with
generally the same interval in the optical axis direction. Also, the
predetermined depth d in the diffraction structure means the length along
the optical axis between the diffraction face of the S+1'th diffraction
face which is formed at the side of the staircase form closest to the
surface (i.e., the highest step, which is the shallowest position) and
diffraction face of the first diffraction face which is formed at the
side of the staircase form closest to the optical element (i.e., the
lowest step, which is the deepest position). This holds true for later
described FIGS. 5B and 5C as well.

[0118]Note that while a structure has been illustrated in FIGS. 5A through
5C wherein the steps of each stepped portion of the staircase shape are
formed such that the closer to the outer side in the radial direction,
the closer to the surface side the steps are formed, but the invention is
not restricted to this arrangement, and an arrangement may be made
wherein the steps of each stepped portion of the diffraction structure
formed of the inner ring zone, middle ring zone, and outer ring zone, are
formed toward the inner side in the radial direction. Specifically,
predetermined diffraction angles and diffraction efficiency can be
obtained by setting the dominant diffraction order and later-described
groove width at each diffraction structure, and also a diffused state or
converged state with a desired diffraction angle can be obtained by
setting the formation direction of the staircase form in accordance with
whether the diffraction order is positive or negative. The symbol Ro
in FIGS. 5A through 5C represents the direction toward the outer side in
the radial direction of the rings, i.e., the direction away from the
optical axis.

[0119]In the first diffraction structure and the later-described second
and third diffraction structures formed at the first diffraction region
51, the groove depth d and number of steps S are determined taking into
consideration the dominant diffraction order and diffraction efficiency.
Also, as shown in FIGS. 5A through 5C, the groove width of each step (the
radial-direction dimension of each step portion of the staircase form) is
such that the steps are formed with equal width within one staircase
form, while looking at the different staircase forms formed continuously
in the radial direction, the value of the step width is smaller at
staircase forms further away form the optical axis. Note that the groove
widths are determined based on phase difference obtained at the
diffraction regions formed with the groove widths, such that the spot
condensed on the signal recording face of the optical disc is optimal.

[0120]For example, the diffraction structure of the first diffraction
region 51 is, as shown in FIG. 5A, a diffraction structure having a
staircase portion including first through fourth steps 51s1, 51s2, 51s3,
and 51s4, formed continuously in the radial direction, wherein the number
of steps is 4 (S=4), and the depth of each step is generally the same
depth (d/4), and first through fifth diffraction faces 51f1, 51f2, 51f3,
51f4, and 51f5 formed at the same intervals of d/4 in the optical axis
direction.

[0121]Also, while description is made here with regard to the first
diffraction region 51 having the cross-sectional form of the rings formed
as a diffraction structure with a multi-step staircase form, any
diffraction structure may be used as long as an optical beam of a
predetermined order is dominant as to the optical beam of each wavelength
as described above, so a configuration may be used such as shown in FIG.
6, with a diffraction region 51B having a diffraction structure wherein
the cross-sectional form of the rings is formed as blazed diffraction
grating having a predetermined depth d, for example.

[0122]Also, in a case wherein the first diffraction region 51 diffracts
the optical beam of the first wavelength which is transmitted
therethrough such that diffracted light of the k1i'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, diffracts
the optical beam of the second wavelength which is transmitted
therethrough such that diffracted light of the k2i'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, and
diffracts the optical beam of the third wavelength which is transmitted
therethrough such that diffracted light of the k3i'th order is dominant,
none of k1i, k2i, and k3i is zero, k1i and k2i are of opposite signs
(k1i×k2i<0), and k2i and k3i are of the same sign
(k2i×k3i>0). Note that in the above case, k1i and k3i are of
opposite signs.

[0123]Now, with the first diffraction region 51, due to the diffraction
order k1i of the first wavelength at which the diffraction efficiency is
maximum being set to other than zero, coupling at the object lens 34 can
be reduced, the problem of noise due to light source return light can be
prevented, and problems such as having to keep the output of the light
source emission within a suitable range with the related art can be
avoided. Also, with the first diffraction region 51, in the event that
the diffraction orders k2i and k3i of the second and third wavelengths at
which the diffraction efficiency is maximum are set to zero, there is no
combination wherein the aberration and efficiency are optimal. In other
words, with the first diffraction region 51, due to the diffraction
orders k2i and k3i being other than zero, a combination can be obtained
wherein aberration and efficiency can be ensured.

[0124]Also, with the first diffraction region 51, due to the relation of
the diffraction orders k1i, k2i, and k3i wherein the diffraction
efficiency of each wavelength is maximum being of a relation wherein k1i
and k2i are of opposite signs, and k2i and k3i are of the same sign,
spherical aberration can be further reduced in a case of condensing
optical beams of each wavelength on the multiple types of optical discs
with the same object lens 34. This is based on the idea that since the
design center of the protective layer is often set to 0.1 to 0.6 in the
event of designing an object lens 34 for the above-described first
through third optical discs, spherical aberration can be suppressed by
inverting the polarity provided to the optical beam of the first
wavelength and the polarity provided to the optical beams of the second
and third wavelengths.

[0125]Further, with the first diffraction region 51, the diffraction
orders k1i, k2i, and k3i of each wavelength wherein the diffraction
efficiency is maximum are set such as to conform to one of the following:

[0127]Specific examples of the first diffraction region 51 which is the
inner ring zone will be given below, with specific numerical values of
the depth d and number of steps S, and the diffraction order of
diffracted light of the order that is dominant in the optical beam of
each wavelength, and the diffraction efficiency of the diffracted light
of each diffraction order is shown in Table 1. Note that Table 1
illustrates Inner Ring Zone Configuration Example 1 through Inner Ring
Zone Configuration Example 4 serving as examples of the first diffraction
region 51, wherein k1i in Table 1 indicates the diffraction order where
the diffraction efficiency of the optical beam of the first wavelength is
maximum, eff1 illustrates the diffraction efficiency of the diffraction
order where the diffraction efficiency of the optical beam of the first
wavelength is maximum, k2i indicates the diffraction order where the
diffraction efficiency of the optical beam of the second wavelength is
maximum, eff2 illustrates the diffraction efficiency of the diffraction
order where the diffraction efficiency of the optical beam of the second
wavelength is maximum, k3i indicates the diffraction order where the
diffraction efficiency of the optical beam of the third wavelength is
maximum, eff3 illustrates the diffraction efficiency of the diffraction
order where the diffraction efficiency of the optical beam of the third
wavelength is maximum, d indicates the groove depth of the first
diffraction region 51, i.e., the distance from the lowest step of the
staircase form to the highest step thereof, and S indicates the number of
steps of the staircase form of the first diffraction region 51.

[0128]Now, the Inner Ring Zone Configuration Example 1 shown in Table 1
will be described. As shown in Table 1, with the Inner Ring Zone
Configuration Example 1, with the groove depth d=3.8 (μm) and the
number of steps S=4, the diffraction efficiency eff1=0.81 for the first
wavelength optical beam diffraction order k1i=+1, the diffraction
efficiency eff2=0.62 for the second wavelength optical beam diffraction
order k2i=-1, and the diffraction efficiency eff3=0.57 for the third
wavelength optical beam diffraction order k3i=-2. This Inner Ring Zone
Configuration Example 1 will be described more specifically with
reference to FIGS. 7A through 7C. FIG. 7A is a diagram illustrating the
change in diffraction efficiency of the +1 order diffracted light of the
optical beam of the first wavelength in a case wherein the groove depth d
is changed in the staircase form with the number of steps S=4, FIG. 7B is
a diagram illustrating the change in diffraction efficiency of the -1
order diffracted light of the optical beam of the second wavelength in a
case wherein the groove depth d is changed in the staircase form with the
number of steps S=4, and FIG. 7C is a diagram illustrating the change in
diffraction efficiency of the -2 order diffracted light of the optical
beam of the third wavelength in a case wherein the groove depth d is
changed in the staircase form with the number of steps S=4. In FIGS. 7A
through 7C, the horizontal axis represents the groove depth in nm, and
the vertical axis represents the diffraction efficiency (intensity of
light). As shown in FIG. 7A, at the position of 3800 nm on the horizontal
axis, eff1 is 0.81, eff2 is 0.62 as shown in FIG. 7B, and eff3 is 0.57 as
shown in FIG. 7C.

[0129]In the same way in Table 1, with the Inner Ring Zone Configuration
Example 2, with the groove depth d=5.3 (μm) and S=6, the diffraction
efficiency eff1, eff2, and eff3 are obtained for the diffraction orders
k1i, k2i, and k3i, as shown in Table 1 and FIGS. 8A through 8C; with the
Inner Ring Zone Configuration Example 3, with the groove depth d=5.1
(μm) and S=5, the diffraction efficiency eff1, eff2, and eff3 are
obtained for the diffraction orders k1i, k2i, and k3i, as shown in Table
1 and FIGS. 9A through 9C; and with the Inner Ring Zone Configuration
Example 4 shown in Table 1 as well, with the groove depth d=5.8 (μm)
and S=6, the diffraction efficiency eff1, eff2, and eff3 are obtained for
the diffraction orders k1i, k2i, and k3i, as shown in Table 1 and FIGS.
10A through 10C.

[0130]The second diffraction region 52 which is a middle ring zone has a
second diffraction structure formed which is ring shaped and has a
predetermined depth, and which is a different structure from the first
diffraction structure. The second diffraction region 52 diffracts the
optical beam of the first wavelength that is transmitted therethrough
such that diffracted light of an order which condenses light so as to
form an appropriate spot on the signal recording face of the first
optical disc via the object lens 34 is dominant, i.e., such that maximum
diffraction efficiency is manifested regarding diffracted light of other
orders.

[0131]The second diffraction region 52 diffracts the optical beam of the
second wavelength that is transmitted therethrough such that diffracted
light of an order which condenses light so as to form an appropriate spot
on the signal recording face of the second optical disc via the object
lens 34 is dominant, i.e., such that maximum diffraction efficiency is
manifested regarding diffracted light of other orders, by way of the
second diffraction structure.

[0132]The second diffraction region 52 diffracts the optical beam of the
third wavelength that is transmitted therethrough such that diffracted
light of orders other than an order which forms an appropriate spot on
the signal recording face of the third optical disc via the object lens
34 is dominant, i.e., such that maximum diffraction efficiency is
manifested regarding diffracted light of other orders, by way of the
second diffraction structure. Note that the second diffraction region 52
can sufficiently reduce diffraction efficiency diffracted light of an
order which forms an appropriate spot on the signal recording face of the
third optical disc via the object lens 34 for the optical beam of the
third wavelength that is transmitted therethrough, by way of the second
diffraction structure.

[0133]Thus, the second diffraction region 52 has a diffraction structure
formed suitable for diffracted light of a predetermined order to be
dominant in the optical beam of each wavelength, thereby enabling
correction and reduction of spherical aberration at the time of optical
beams of first and second wavelengths that have passed through the second
diffraction region 52 and become diffracted light of a predetermined
order being condensed on the signal recording face of the respective
optical discs by the object lens 34.

[0134]Also, the second diffraction region 52 is configured so as to
function as described above regarding the optical beams of the first and
second wavelengths, but for the optical beam of the third wavelength such
that diffracted light of orders other than diffracted light of an order
which is condensed on the signal recording face of the third optical disc
after passing through the second diffraction region 52 and the object
lens 34 is dominant, whereby aperture restriction can be applied to the
optical beam of the third wavelength, such that there is very little
effect even if the optical beam of the third wavelength which has been
transmitted through the second diffraction region 52 is input to the
object lens 34, there is hardly any effect on the signal recording face
of the third optical disc, i.e., markedly reducing the light quantity of
the optical beam of the third wavelength which is condensed on the signal
recording face after passing through the second diffraction region 52 and
the object lens 34, to around zero.

[0135]Now, the above-described first diffraction region 51 is formed of a
size such that the optical beam of the third wavelength which has been
transmitted through the region thereof is input to the object lens 34 in
the same state as an optical beam which has been subjected to aperture
restriction at around NA=0.45, and since the second diffraction region 52
formed on the outer side of the first diffraction region 51 does not
allow condensation of the optical beam of the third wavelength which has
been transmitted through this region on the third optical disc via the
optical lens 34, the diffraction unit 50 which has the first and second
diffraction regions 51 and 52 configured thus functions so as to restrict
the numerical aperture of the optical beam of the third wavelength to
around NA=0.45. It should be noted however, that while in this
arrangement of the diffraction unit 50, the optical beam of the third
wavelength is subjected to aperture restriction around NA=0.45, but the
present invention is not restricted to this, i.e., numerical aperture
restriction due to the above configuration is not limited to this.

[0136]Specifically, as shown in FIGS. 4 and 5B, in the same way as with
the above-described first diffraction region 51, the second diffraction
region 52 is formed with the cross-sectional form of ring shapes centered
on the optical axis being formed in a staircase-like shape having a
predetermined depth d and a predetermined number of steps S, continuing
in the radial direction in a staircase form. Note that the values of the
second diffraction region 52 for d and/or S differ from those with the
first diffraction region 51, so the second diffraction region 52 has
formed a second diffraction structure which differs from the diffraction
structure formed with the first diffraction region 51. For example, the
diffraction structure of the second diffraction region 52 is, as shown in
FIG. 5B, a diffraction structure having a staircase portion including
first through third steps 52s1, 52s2, and 52s3, formed continuously in
the radial direction, wherein the number of steps is 3 (S=3), and the
depth of each step is generally the same depth (d/3), and first through
fourth diffraction faces 52f1, 52f2, 52f3, and 52f4 formed at the same
intervals of d/3 in the optical axis direction.

[0137]Also, while description is made here with regard to the second
diffraction region 52 having the cross-sectional form of the rings formed
as a diffraction structure with a multi-step staircase form, any
diffraction structure may be used as long as an optical beam of a
predetermined order is dominant as to the optical beam of each wavelength
as described above, in the same way as with the first diffraction region
51, so a configuration may be used such as shown in FIG. 6, with a
diffraction region 52B having a diffraction structure wherein the
cross-sectional form of the rings is formed as blazed diffraction grating
having a predetermined depth d, for example.

[0138]Also, in a case wherein the second diffraction region 52 diffracts
the optical beam of the first wavelength which is transmitted
therethrough such that diffracted light of the k1m'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, and
diffracts the optical beam of the second wavelength which is transmitted
therethrough such that diffracted light of the k2m'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, the
diffraction orders k1m and k2m are set such as to conform to one of the
following:

[0140]Specific examples of the second diffraction region 52 which is the
middle ring zone will be given below, with specific numerical values of
the depth d and number of steps S, and the diffraction order of
diffracted light of the order that is dominant in the optical beam of
each wavelength, and the diffraction efficiency of the diffracted light
of each diffraction order is shown in Table 2. Note that Table 2
illustrates Middle Ring Zone Configuration Example 1 through Middle Ring
Zone Configuration Example 3, wherein k1m in Table 2 indicates the
diffraction order where the diffraction efficiency of the optical beam of
the first wavelength is maximum, eff1 illustrates the diffraction
efficiency of the diffraction order where the diffraction efficiency of
the optical beam of the first wavelength is maximum, k2m indicates the
diffraction order where the diffraction efficiency of the optical beam of
the second wavelength is maximum, eff2 illustrates the diffraction
efficiency of the diffraction order where the diffraction efficiency of
the optical beam of the second wavelength is maximum, k3m indicates the
diffraction order where the optical beam of the third wavelength is
selected as described below, eff3 illustrates the diffraction efficiency
of the diffraction order where the optical beam of the third wavelength
is selected, d indicates the groove depth of the second diffraction
region 52, i.e., the distance from the lowest step of the staircase form
to the highest step thereof, and S indicates the number of steps of the
staircase form of the second diffraction region 52. Note that the
asterisks in Table 2 indicate diffraction order for condensing an optical
beam passing through the middle ring zone in this configuration example
so as to appropriately form a spot on the signal recording face of the
corresponding optical disk via the object lens 34, i.e., a diffraction
order whereby spherical aberration on the signal recording face of the
corresponding optical disc can be corrected, and "≈0" indicates
that the diffraction efficiency is at a state of approximately zero.

[0141]Now, the Middle Ring Zone Configuration Example 1 shown in Table 2
will be described. As shown in Table 2, with the Middle Ring Zone
Configuration Example 1, with the groove depth d=8.6 (μm) and the
number of steps S=3, the diffraction efficiency eff1=0.76 for the first
wavelength optical beam diffraction order k1m=-1, the diffraction
efficiency eff2=0.77 for the second wavelength optical beam diffraction
order k2m=+1. Also, the diffraction efficiency eff3 is approximately 0
for the diffraction order k3m, where optical beams of the third
wavelength passing through this region are condensed on the signal
recording face of the third optical disc so as to form a spot, via the
object lens 34.

[0142]This Middle Ring Zone Configuration Example 1 will be described more
specifically with reference to FIGS. 11A through 11C. FIG. 11A is a
diagram illustrating the change in diffraction efficiency of the -1 order
diffracted light of the optical beam of the first wavelength in a case
wherein the depth d is changed in the staircase form with the number of
steps S=3, FIG. 11B is a diagram illustrating the change in diffraction
efficiency of the +1 order diffracted light of the optical beam of the
second wavelength in a case wherein the depth d is changed in the
staircase form with the number of steps S=3, and FIG. 11C is a diagram
illustrating the change in diffraction efficiency of the +2 order
diffracted light of the optical beam of the third wavelength in a case
wherein the depth d is changed in the staircase form with the number of
steps S=3. In FIGS. 11A through 11C, the horizontal axis represents the
groove depth in nm, and the vertical axis represents the diffraction
efficiency (intensity of light). As shown in FIG. 11A, at the position of
8600 nm on the horizontal axis, eff1 is 0.76, eff2 is 0.77 as shown in
FIG. 11B, and eff3 is approximately zero as shown in FIG. 11C. The
diffraction order k3m of the optical beam of the third wavelength noted
by the asterisk in Table 2 is k3m=+2.

[0143]In the same way in Table 2, with the Middle Ring Zone Configuration
Example 2, with the groove depth d=14.8 (μm) and S=5, the diffraction
efficiency eff1, eff2, and eff3 are obtained for the diffraction orders
k1m, k2m, and k3m, as shown in Table 2 and FIGS. 12A through 12C; and
with the Middle Ring Zone Configuration Example 3 shown in Table 2, with
the groove depth d=14.1 (μm) and S=5, the diffraction efficiency eff1,
eff2, and eff3 are obtained for the diffraction orders k1m, k2m, and k3m,
as shown in Table 2 and FIGS. 13A through 13C.

[0144]The third diffraction region 53 which is an outer ring zone has a
third diffraction structure formed which is ring shaped and has a
predetermined depth, and which is a different structure from the first
and second diffraction structures. The third diffraction region 53
diffracts the optical beam of the first wavelength that is transmitted
therethrough such that diffracted light of an order which forms an
appropriate spot condensed on the signal recording face of the first
optical disc via the object lens 34 is dominant, i.e., such that maximum
diffraction efficiency is manifested regarding diffracted light of other
orders.

[0145]The third diffraction region 53 diffracts the optical beam of the
second wavelength that is transmitted therethrough such that diffracted
light of orders other than an order which forms an appropriate spot
condensed on the signal recording face of the second optical disc via the
object lens 34 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders, by
way of the third diffraction structure. Note that the third diffraction
region 53 can sufficiently reduce diffraction efficiency diffracted light
of an order which forms an appropriate spot condensed on the signal
recording face of the second optical disc via the object lens 34 for the
optical beam of the second wavelength that is transmitted therethrough,
by way of the third diffraction structure.

[0146]The third diffraction region 53 diffracts the optical beam of the
third wavelength that is transmitted therethrough such that diffracted
light of orders other than an order which forms an appropriate spot
condensed on the signal recording face of the third optical disc via the
object lens 34 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders, by
way of the third diffraction structure. Note that the third diffraction
region 53 can sufficiently reduce diffraction efficiency diffracted light
of an order which forms an appropriate spot condensed on the signal
recording face of the third optical disc via the object lens 34 for the
optical beam of the third wavelength that is transmitted therethrough, by
way of the third diffraction structure.

[0147]Thus, the third diffraction region 53 has a diffraction structure
suitably formed whereby diffracted light of a predetermined order is
dominant in the optical beam of each wavelength, thereby enabling
correction and reduction of spherical aberration at the time of optical
beams of the first wavelength that has passed through the third
diffraction region 53 and become diffracted light of a predetermined
order being condensed on the signal recording face of the respective
optical discs by the object lens 34.

[0148]Also, the third diffraction region 53 is configured so as to
function as described above regarding the optical beams of the first
wavelength, but such that regarding the second and third wavelength
beams, diffracted light of orders other than diffracted light of an order
which is condensed on the signal recording face of the second and third
optical discs after passing through the third diffraction region 53 and
the object lens 34 is dominant, whereby aperture restriction can be
applied to the optical beam of the second wavelength, such even if the
optical beams of the second and third wavelengths which have been
transmitted through the third diffraction region 53 are input to the
object lens 34, that there is very little effect on the signal recording
face of the second and third optical discs, i.e., markedly reducing the
light quantity of the optical beams of the second and third wavelengths
which are condensed on the signal recording faces after passing through
the third diffraction region 53 and the object lens 34, to around zero.
Also, the third diffraction region 53 can function to subject the optical
beam of the third wavelength to aperture restriction along with the
above-described second diffraction region 52.

[0149]Now, the above-described second diffraction region 52 is formed of a
size such that the optical beam of the second wavelength which has been
transmitted through the region thereof is input to the object lens 34 in
the same state as an optical beam which has been subjected to aperture
restriction at around NA=0.6, and since the third diffraction region 53
formed on the outer side of the second diffraction region 52 does not
allow condensation of the optical beam of the second wavelength which has
been transmitted through this region on the optical disc via the object
lens 34, the diffraction unit 50 which has the second and third
diffraction regions 52 and 53 configured thus functions so as to restrict
the numerical aperture of the optical beam of the second wavelength to
around NA=0.6. It should be noted however, that while in this arrangement
of the diffraction unit 50, the optical beam of the second wavelength is
subjected to aperture restriction around NA=0.6, but the present
invention is not restricted to this, i.e., numerical aperture restriction
due to the above configuration is not limited to this.

[0150]Also, the third diffraction region 53 is formed of a size such that
the optical beam of the first wavelength which has been transmitted
through the region thereof is input to the object lens 34 in the same
state as an optical beam which has been subjected to aperture restriction
at around NA=0.85, and since there is no diffraction structure formed on
the outer side of the third diffraction region 53, this does not allow
condensation of the optical beam of the first wavelength which has been
transmitted through this region on the first optical disc via the object
lens 34, and the diffraction unit 50 which has the third diffraction
region 53 configured thus functions so as to restrict the numerical
aperture of the optical beam of the first wavelength to around NA=0.85.
Note that with the first wavelength optical beam transmitted through the
third diffraction region 53, light of diffraction orders of -1, +1, +2,
and -2 is dominant, so the zero-order light transmitted through the
region outside the third diffraction region 53 almost never passes
through the object lens 34 to be condensed on the first optical disc, but
in cases wherein this zero-order does pass through the object lens 34 and
is condensed on the first optical disc, a configuration may be provided
to perform aperture restriction by providing, at the region outside of
the third diffraction region 53, either a shielding portion for shielding
optical beams passing through, or a diffraction region having a
diffraction structure wherein optical beams of orders other than the
order of the optical beam passing through the object lens 34 to be
condensed on the first optical disc are dominant. It should be noted
however, that while in this arrangement of the diffraction unit 50, the
optical beam of the first wavelength is subjected to aperture restriction
around NA=0.85, but the present invention is not restricted to this,
i.e., numerical aperture restriction due to the above configuration is
not limited to this.

[0151]Specifically, as shown in FIGS. 4 and 5C, in the same way as with
the above-described first diffraction region 51, the third diffraction
region 53 is formed with the cross-sectional form of ring shapes centered
on the optical axis being formed in a staircase-like shape having a
predetermined depth d and a predetermined number of steps S, continuing
in the radial direction in a staircase form. Note that the values of the
third diffraction region 53 for d and/or S differ from those with the
first and second diffraction regions 51 and 52, so the third diffraction
region 53 has formed a third diffraction structure which differs from the
first and second diffraction structures formed with the first and second
diffraction regions 51 and 52. For example, the diffraction structure of
the third diffraction region 53 is, as shown in FIG. 5C, a diffraction
structure having a staircase portion including first and second steps
53s1 and 53s2, formed continuously in the radial direction, wherein the
number of steps is 2 (S=2), and the depth of each step is generally the
same depth (d/2), and first through third diffraction faces 53f1, 53f2,
and 53f3 formed at the same intervals of d/2 in the optical axis
direction.

[0152]Also, while description is made here with regard to the third
diffraction region 53 having the cross-sectional form of the rings formed
as a diffraction structure with a multi-step staircase form, any
diffraction structure may be used as long as an optical beam of a
predetermined order is dominant as to the optical beam of each wavelength
as described above, in the same way as with the first and second
diffraction regions 51 and 52, so a configuration may be used such as
shown in FIG. 6, with a diffraction region 53B having a diffraction
structure wherein the cross-sectional form of the rings is formed as a
blazed form having a predetermined depth d, for example.

[0153]Specific examples of the third diffraction region 53 which is the
outer ring zone will be given below, with specific numerical values of
the depth d and number of steps S, and the diffraction order of
diffracted light of the order that is dominant in the optical beam of
each wavelength, and the diffraction efficiency of the diffracted light
of each diffraction order is shown in Table 3. Note that Table 3
illustrates Outer Ring Zone Configuration Example 1 through Outer Ring
Zone Configuration Example 4, wherein k1o in Table 3 indicates the
diffraction order where the diffraction efficiency of the optical beam of
the first wavelength is maximum, eff1 illustrates the diffraction
efficiency of the diffraction order where the diffraction efficiency of
the optical beam of the first wavelength is maximum, k2o indicates the
diffraction order where the optical beam of the second wavelength is
selected as described below, eff2 illustrates the diffraction efficiency
of the diffraction order where the optical beam of the second wavelength
is selected, k3o indicates the diffraction order where the optical beam
of the third wavelength is selected as described below, eff3 illustrates
the diffraction efficiency of the diffraction order where the optical
beam of the third wavelength is selected, d indicates the groove depth of
the third diffraction region 53, i.e., the distance from the lowest step
of the staircase form to the highest step thereof, and S indicates the
number of steps of the staircase form of the third diffraction region 53.
Note that the asterisks in Table 3 indicate diffraction order for
condensing an optical beam passing through the outer ring zone in this
configuration example so as to appropriately form a spot on the signal
recording face of the corresponding optical disk via the object lens 34,
i.e., a diffraction order whereby spherical aberration on the signal
recording face of the corresponding optical disc can be corrected, and
"≈0" indicates that the diffraction efficiency is at a state of
approximately zero.

[0154]Now, the Outer Ring Zone Configuration Example 1 shown in Table 3
will be described. As shown in Table 3, with the Outer Ring Zone
Configuration Example 1, with the groove depth d=4.2 (μm) and the
number of steps S=2, the diffraction efficiency eff1=0.63 for the first
wavelength optical beam diffraction order k1o=-1. Also, the diffraction
efficiency eff2 is approximately 0 for the second wavelength optical beam
diffraction order k2o, where optical beams of the second wavelength
passing through this region are condensed on the signal recording face of
the second optical disc so as to form a spot, via the object lens 34.
Further, the diffraction efficiency eff3 is approximately 0 for the third
wavelength optical beam diffraction order k3o, where optical beams of the
third wavelength passing through this region are condensed on the signal
recording face of the third optical disc so as to form a spot, via the
object lens 34.

[0155]Next, this Outer Ring Zone Configuration Example 1 will be described
more specifically with reference to FIGS. 14A through 14C. FIG. 14A is a
diagram illustrating the change in diffraction efficiency of the -1 order
diffracted light of the optical beam of the first wavelength in a case
wherein the depth d is changed in the staircase form with the number of
steps S=2, FIG. 14B is a diagram illustrating the change in diffraction
efficiency of the +1 order diffracted light of the optical beam of the
second wavelength in a case wherein the depth d is changed in the
staircase form with the number of steps S=2, and FIG. 14C is a diagram
illustrating the change in diffraction efficiency of the +2 order
diffracted light of the optical beam of the third wavelength in a case
wherein the depth d is changed in the staircase form with the number of
steps S=2. In FIGS. 14A through 14C, the horizontal axis represents the
groove depth in nm, and the vertical axis represents the diffraction
efficiency (intensity of light). As shown in FIG. 14A, at the position of
4200 nm on the horizontal axis, eff1 is 0.63, eff2 is approximately zero
as shown in FIG. 14B, and eff3 is approximately zero as shown in FIG.
14C. The diffraction orders k2o and k3o noted by the asterisks in Table 3
are k2o=+1 and k3o=+2.

[0156]In the same way with the Outer Ring Zone Configuration Example 2 in
Table 3, with the groove depth d=0.5 (μm) and S=5, the diffraction
efficiency eff1, eff2, and eff3 are obtained for the diffraction orders
k1o, k2o, and k3o, as shown in Table 3 and FIGS. 15A through 15C; with
the Outer Ring Zone Configuration Example 3 in Table 3, with the groove
depth d=1.2 (μm) and S=5, the diffraction efficiency eff1, eff2, and
eff3 are obtained for the diffraction orders k1o, k2o, and k3o, as shown
in Table 3 and FIGS. 16A through 16C; and with the Outer Ring Zone
Configuration Example 4 in Table 3, with the groove depth d=6.4 (μm)
and S=5, the diffraction efficiency eff1, eff2, and eff3 are obtained for
the diffraction orders k1o, k2o, and k3o, as shown in Table 3 and FIGS.
17A through 17C.

[0157]The diffraction unit 50, having the first through third diffraction
regions 51, 52, and 53 with the configuration such as described above, is
capable of condensation of the optical beams of the first through third
wavelengths passing through the first diffraction region 51 so as to form
a suitable spot on the signal recording face of the corresponding optical
disc by being input to the object lens 34, in a divergent angle state
wherein no spherical aberration occurs at the signal recording face of
respectively corresponding optical discs via the common object lens 34,
i.e., in a dispersed state or converged state wherein spherical
aberration is corrected via the object lens 34, and is capable of
condensation of the optical beams of the first and second wavelengths
passing through the second diffraction region 52 so as to form a suitable
spot on the signal recording face of the corresponding optical disc by
being input to the object lens 34, in a divergent angle state wherein no
spherical aberration occurs at the signal recording face of respectively
corresponding optical discs via the common object lens 34, i.e., in a
dispersed state or converged state wherein spherical aberration is
corrected via the object lens 34, and also is capable of condensation of
the optical beam of the first wavelength passing through the third
diffraction region 53 so as to form a suitable spot on the signal
recording face of the corresponding optical disc by being input to the
object lens 34, in a divergent angle state wherein no spherical
aberration occurs at the signal recording face of the corresponding
optical disc via the object lens 34, i.e., in a dispersed state or
converged state wherein spherical aberration is corrected via the object
lens 34.

[0158]That is to say, the diffraction unit 50 provided on one face of the
diffraction optical element 35 disposed on the optical path between the
first through third emitting units of the optical pickup 3 and the signal
recording face allows optical beams of respective wavelengths passing
through the respective regions (first through third diffraction regions
51, 52, and 53) to be input to the object lens 34 in a state wherein
spherical aberration occurring at the signal recording face to be
reduced, so spherical aberration occurring at the signal recording face
when condensing optical beams of the first through third wavelengths on
the signal recording face of the respective corresponding optical discs
using the common object lens 34 in the optical pickup 3 can be minimized,
which is to say that three-wavelength compatibility of the optical pickup
using three types of wavelengths for three types of optical discs and a
common object lens 34 can be realized, wherein information signals can be
recorded to and/or played from respective optical discs.

[0159]Also, the diffraction unit 50 having the first through third
diffraction regions 51, 52, and 53 performs diffraction of the optical
beam of the third wavelength passing through the second and third
diffraction regions 52 and 53 such that an order other than the
diffraction order where the optical beam is appropriately condensed on
the signal recording face of the corresponding type of optical disc via
the object lens 34 is dominant, whereby, with regard to the optical beam
of the third wavelength, only the optical beam portion which has passed
through the first diffraction region 51 is condensed on the signal
recording face of the optical disc via the object lens 34, and also, the
first diffraction region 51 is formed to a size which is the
predetermined numerical aperture of the third wavelength optical beam
passing through this region, whereby aperture restriction can be
performed regarding the optical beam of the third wavelength such that
NA= around 0.45, for example.

[0160]Also, the diffraction unit 50 performs diffraction of the optical
beam of the second wavelength passing through the third diffraction
region 53 such that an order other than the diffraction order where the
optical beam is appropriately condensed on the signal recording face of
the corresponding type of optical disc via the object lens 34 is
dominant, whereby, with regard to the optical beam of the second
wavelength, only the optical beam portion which has passed through the
first and second diffraction regions 51 and 52 is condensed on the signal
recording face of the optical disc via the object lens 34, and also, the
first and second diffraction regions 51 and 52 are formed to a size which
is the predetermined numerical aperture of the second wavelength optical
beam passing through this region, whereby aperture restriction can be
performed regarding the optical beam of the second wavelength such that
NA= around 0.60, for example.

[0161]Also, the diffraction unit 50 places the optical beam of the first
wavelength passing outside of the third diffraction region 53 in a state
so as to not be suitably condensed on the signal recording face of the
corresponding type of optical disc via the object lens 34, or shields the
optical beam of the first wavelength passing outside of the third
diffraction region 53, whereby, with regard to the optical beam of the
first wavelength, only the optical beam portion which has passed through
the first through third diffraction regions 51, 52, and 53 is condensed
on the signal recording face of the optical disc via the object lens 34,
and also, the first through third diffraction regions 51, 52, and 53 are
formed to a size which is the predetermined numerical aperture of the
first wavelength optical beam passing through this region, whereby
aperture restriction can be performed regarding the optical beam of the
first wavelength such that NA= around 0.85, for example.

[0162]Thus, the diffraction unit 50 provided on one face of the
diffraction optical element 35 disposed on the optical path as described
above not only realizes three-wavelength compatibility, but also enables
optical beams of each wavelength to be input to the common object lens 34
in a state wherein aperture restriction is performed appropriately for
each of the three types of optical discs and optical beams of the first
through third wavelengths. That is to say, the diffraction unit 50 not
only has functions of aberration correction corresponding to three
wavelengths, but also has functions as an aperture restricting unit.

[0163]It should be noted that a diffraction unit can be configured by
suitably combining the above-described diffraction region examples. That
is to say, the diffraction order of each wavelength passing through each
diffraction region can be selected as appropriate. In the event of
changing the diffraction order of each wavelength passing through each
diffraction region, an object lens 34 corresponding to each diffraction
order of each wavelength passing through each diffraction region can be
used.

[0164]Also, while the first through third diffraction regions 51, 52, and
53 have been shown here having a so-called multi-step form diffraction
structure with a staircase form having steps of a predetermined depth, a
configuration may be used such as shown in FIG. 6, formed as a blazed
form. Particularly, with a diffraction region having a diffraction
structure with a shallow groove depth d formed, such as the third
diffraction region, the manufacturing processes is simplified by forming
as a blazed form, thereby simplifying and reducing costs of
manufacturing.

[0165]Also, while description has been made above with the diffraction
unit 50 configured of the three diffraction regions 51, 52, and 53 formed
on the incident side face of the diffraction optical element 35 provided
separately from the object lens 34, as shown in FIG. 18A, the present
invention is not restricted to this arrangement, and may be provided to
the output side face of the diffraction optical element 35. Further, the
diffraction unit 50 having the first through third diffraction regions
51, 52, and 53, can be integrally configured on the input or output side
of the object lens 34, or further, as shown in FIG. 18B for example, an
object lens 34B having the diffraction unit 50 on the incident side
thereof may be configured. In the event of providing the diffraction unit
50 on the incident side face of the object lens 34B for example, the
planar shape of the above-described diffraction structure is combined
with a reference face at the incident side required for the lens to be
able to function as an object lens. While the above-described diffraction
optical element 35 and the object lens 34 are two separate elements
serving as a condensing optical device, the object lens 34B thus
configured functions as a condensing optical device which can perform
suitable light condensing such that spherical aberration does not occur
at the signal recording face of optical discs corresponding to each of
the three optical beams of different wavelengths, with a single element.
Providing the diffraction unit 50 integrally with the object lens 34B
enables further reduction in optical parts and also reduction in
configuration size.

[0166]The object lens 34B having a diffraction unit having functions the
same as the diffraction unit 50 provided integrally at the input side or
output side face realizes three-wavelength compatibility of the optical
pickup 3 by reducing aberration and so forth when used in an optical
pickup, and also reduces the number of parts so as to enable
simplification and reduction in size of the configuration, thereby
realizing high production and reduced costs. Note that the
above-described diffraction unit 50 sufficiently manifests the advantages
thereof with the diffraction structure for aberration correction to
realize three-wavelength compatibility being provided on a single face
that has been difficult with the related art, which enables such a
diffraction element to be integrally formed with the object lens 34,
further enabling directly forming a diffraction face on a plastic lens,
and forming the object lens 34B with which the diffraction unit 50 has
been integrated of a plastic material further realized improved
production and lower costs.

[0167]The collimator lens 42 provided between the diffraction optical
element 35 and the third beam splitter 38 converts the divergent angle of
the first through third wavelength optical beams of which the optical
paths have been synthesized at the second beam splitter 37 and passed
through the third beam splitter, and outputs to the quarter-wave plate 43
and diffraction optical element 35 side, in a generally parallel light
state, for example. The arrangement wherein the collimator lens 42 inputs
the optical beams of the first and second wavelengths into the
above-described diffraction optical element 35 with the divergent angle
thereof in the state of generally parallel light, and also inputs the
optical beam of the third wavelength into the diffraction optical element
35 in a divergent angle state which is slightly diffused or converged as
to parallel light (hereinafter also referred to as "finite system state")
enables further reduction of spherical aberration at the time of
condensing the third wavelength optical beam on the signal recording face
of the third optical disc via the diffraction optical element 35 and the
object lens 34. While an arrangement has been described here wherein the
optical beam of the third wavelength is input to the diffraction optical
element 35 in a state of a predetermined divergent angle, due to the
positional relation between the third light source 33 having the third
emitting unit for emitting the third wavelength optical beam and the
collimator lens 42, in the event of positioning multiple emitting units
at a common light source for example, this may be realized by providing
an element which converts only the divergent angle of the optical beam of
the third wavelength, or by inputting into the diffraction optical
element 35 in a predetermined divergent angle state by providing a
mechanism to drive the collimator lens 42. Also, the optical beams of the
second wavelength, or the optical beams of the second and third
wavelengths, may be input to the diffraction optical element 35 in the
finite system state, thereby further reducing aberration.

[0168]The multi-lens 46 is, for example, a wavelength-selective
multi-lens, whereby the returning first through third wavelength optical
beams separated from the outgoing path optical beams by being reflected
at the third beam splitter 38, after having been reflected off of the
signal recording face of the respective optical disc, and passed through
the object lens 34, diffraction optical element 35, redirecting mirror
44, quarter-wave plate 43, and collimator lens 42, is appropriately
condensed on the photoreception face of the photodetector or the like of
the photosensor 45. At this time, the multi-lens 46 provides the return
optical beam with astigmatism for detection of focus error signals or the
like.

[0169]The photosensor 45 receives the return optical beam condensed at the
multi-lens 46, and detects, along with information signals, various types
of detection signals such as focus error signals, tracking error signals,
and so forth.

[0170]With the optical pickup 3 configured as described above, the object
lens 34 is driven so as to be displaced based on the focus error signals
and tracking error signals obtained by the photosensor 45, whereby the
object lens 34 is moved to a focal position as to the signal recording
face of the optical disc 2, the optical beam is focused onto the signal
recording face of the optical disc 2, and information is recorded to or
played from the optical disc 2.

[0171]The optical pickup 3 is provided on one face of the diffraction
optical element 35, can provide optical beams of each wavelength with a
diffraction efficiency and diffraction angle suitable for each region due
to the diffraction unit 50 having the first through third diffraction
regions 51, 52, and 53, can sufficiently reduce spherical aberration at
the signal recording face of the three types of first through third
optical discs 11, 12, and 13, of which the format for the thickness of
the protective layer differs, and enables reading and writing of signals
to and from the multiple types of optical discs 11, 12, and 13, using
optical beams of three different wavelengths.

[0172]Also, the diffraction optical element 35 having the diffraction unit
50, and object lens 34, in the above optical pickup 3, can function as a
condensing optical device for condensing incident optical beams at a
predetermined position. In the event of using an optical pickup which
performs recording and/or playing of information signals by irradiating
optical beams onto three different types of optical discs, the
diffraction unit 50 provided on one face of the diffraction optical
element 35 enables the condensing optical device to appropriately
condense corresponding optical beams onto the signal recording face of
the three types of optical discs in a state with spherical aberration
sufficiently reduced, meaning that three-wavelength compatibility of the
optical pickup using the object lens 34 common to the three wavelengths
can be realized.

[0173]Also, while description has been made above regarding a
configuration wherein the diffraction optical element 35 to which the
diffraction unit 50 is provided, and the object lens 34, are provided to
an actuator such as an object lens driving mechanism or the like for
driving the object lens 34 is as to be integral, this may be configured
as a condensing optical unit wherein the diffraction optical element 35
and the object lens 34 are formed as an integrated unit, in order to
improve precision of assembly to the lens holder of the actuator, and
facilitate assembly work. For example, a condensing optical unit can be
configured by use of spacers or the like to fix the diffraction optical
element 35 and object lens 34 to the holder while setting the
positioning, spacing, and optical axis, so as to be integrally formed.
Due to being integrally assembled to the object lens driving mechanism as
described above, the diffraction optical element 35 and object lens 34
can appropriately condense the first through third wavelength optical
beams on the signal recording face of the respective optical discs in a
state with spherical aberration reduced, even at the time of field shift
such as displacement in the tracking direction.

[0174]Next, the optical paths of the optical beams emitted from the first
through third light sources 31, 32, and 33 of the optical pickup 3
configured as described above, will be described with reference to FIG.
2. First, the optical path at the time of emitting the optical beam of
the first wavelength as to the first optical disc 11 and performing
reading or writing of information will be described.

[0175]The disc type determination unit 22 which has determined that the
type of the optical disc 2 is the first optical disc 11 causes the
optical beam of the first wavelength to be emitted from the first
emitting unit of the first light source 31.

[0176]The optical beam of the first wavelength emitted form the first
emitting unit is split into three beams by the first grating 39, for
detection of tracking error signals and so forth, and is input to the
second beam splitter 37. The optical beam of the first wavelength which
has been input to the second beam splitter 37 is reflected at a mirror
face 37a thereof, and is output to the third beam splitter 38 side.

[0177]The optical beam of the first wavelength which is input to the third
beam splitter 38 is transmitted through a mirror face 38a thereof, output
to the collimator lens 42 side, where the divergent angle is converted by
the collimator lens 42 so as to be generally parallel light, provided
with a predetermined phase difference at the quarter-wave plate 43,
reflected off of the redirecting mirror 44, and output to the diffraction
optical element 35 side.

[0178]The optical beam of the first wavelength which is input to the
diffraction optical element 35 is output with the optical beam which has
passed through each region thereof having a predetermined diffraction
order dominant therein as described above, due to the first through third
diffraction regions 51, 52, and 53 of the diffraction unit 50 provided on
the incident side face thereof, and input to the object lens 34. The
optical beam of the first wavelength output from the diffraction optical
element 35 is not only in a state of a predetermined divergent angle, but
also is in a state of aperture restriction.

[0179]The optical beam of the first wavelength input to the object lens 34
has been input in a divergent angle state whereby spherical aberration of
the optical beam having passed through the regions 51, 52, and 53 can be
reduced, and accordingly is appropriately condensed by the object lens 34
on the signal recording face of the first optical disc 11.

[0180]The optical beam condensed at the first optical disc 11 is reflected
at the signal recording face, passes through the object lens 34,
diffraction optical element 35, redirecting mirror 44, quarter-wave plate
43, and collimator lens 42, is reflected off of the mirror face 38a of
the third beam splitter 38, and is output to the photosensor 45 side.

[0181]The optical beam split from the optical path of the outgoing optical
beam reflected off of the third beam splitter 38 is condensed on the
photoreception face of the photosensor 45 by the multi-lens 46, and
detected.

[0182]Next, description will be made regarding the optical path at the
time of emitting an optical beam of the second wavelength to the second
optical disc 12 and reading or writing information. The disc type
determination unit 22 which has determined that the type of the optical
disc 2 is the second optical disc 12 causes the optical beam of the
second wavelength to be emitted from the second emitting unit of the
second light source 32.

[0183]The optical beam of the second wavelength emitted from the second
emitting unit is split into three beams by the second grating 40, for
detection of tracking error signals and so forth, and is input to the
first beam splitter 36. The optical beam of the second wavelength which
has been input to the first beam splitter 36 is transmitted through a
mirror face 36a thereof, also transmitted through the mirror face 37a of
the second beam splitter 37, and is output to the third beam splitter 38
side.

[0184]The optical beam of the second wavelength which is input to the
third beam splitter 38 is transmitted through the mirror face 38a
thereof, output to the collimator lens 42 side, where the divergent angle
is converted by the collimator lens 42 so as to be generally parallel
light, provided with a predetermined phase difference at the quarter-wave
plate 43, reflected off of the redirecting mirror 44, and output to the
diffraction optical element 35 side.

[0185]The optical beam of the second wavelength which is input to the
diffraction optical element 35 is output with the optical beam which has
passed through each region thereof having a predetermined diffraction
order dominant therein as described above, due to the first through third
diffraction regions 51, 52, and 53 of the diffraction unit 50 provided on
the incident side face thereof, and input to the object lens 34. The
optical beam of the second wavelength output from the diffraction optical
element 35 is not only in a state of a predetermined divergent angle, but
also is in a state of aperture restriction due to entering the object
lens 34.

[0186]The optical beam of the second wavelength input to the object lens
34 has been input in a divergent angle state whereby spherical aberration
of the optical beams having passed through the first and second
diffraction regions 51 and 52 can be reduced, and accordingly is
appropriately condensed by the object lens 34 on the signal recording
face of the second optical disc 12.

[0187]The return optical path of the optical beam reflected off of the
signal recording face of the second optical disc 12 is the same as with
the case of the above-described optical beam of the first wavelength, and
accordingly description thereof will be omitted.

[0188]Next, description will be made regarding the optical path at the
time of emitting an optical beam of the third wavelength to the third
optical disc 13 and reading or writing information. The disc type
determination unit 22 which has determined that the type of the optical
disc 2 is the third optical disc 13 causes the optical beam of the third
wavelength to be emitted from the third emitting unit of the third light
source 33.

[0189]The optical beam of the third wavelength emitted form the third
emitting unit is split into three beams by the third grating 41, for
detection of tracking error signals and so forth, and is input to the
first beam splitter 36. The optical beam of the third wavelength which
has been input to the first beam splitter 36 is reflected off of the
mirror face 36a thereof, transmitted through the mirror face 37a of the
second beam splitter 37, and is output to the third beam splitter 38
side.

[0190]The optical beam of the third wavelength which is input to the third
beam splitter 38 is transmitted through the mirror face 38a thereof,
output to the collimator lens 42 side, where the divergent angle is
converted by the collimator lens 42 so as to be diffused or converged as
to generally parallel light, provided with a predetermined phase
difference at the quarter-wave plate 43, reflected off of the redirecting
mirror 44, and output to the diffraction optical element 35 side.

[0191]The optical beam of the third wavelength which is input to the
diffraction optical element 35 is output with the optical beam which has
passed through each region thereof having a predetermined diffraction
order dominant therein as described above, due to the first through third
diffraction regions 51, 52, and 53 of the diffraction unit 50 provided on
the incident side face thereof, and input to the object lens 34. The
optical beam of the third wavelength output from the diffraction optical
element 35 is not only in a state of a predetermined divergent angle, but
also is in a state of aperture restriction due to having been input to
the object lens 34.

[0192]The optical beam of the third wavelength input to the object lens 34
has been input in a divergent angle state whereby spherical aberration of
the optical beam having passed through the first diffraction region 51
can be reduced, and accordingly is appropriately condensed by the object
lens 34 on the signal recording face of the third optical disc 13.

[0193]The return optical path of the optical beam reflected off of the
signal recording face of the third optical disc 13 is the same as with
the case of the above-described optical beam of the first wavelength, and
accordingly description thereof will be omitted.

[0194]Note that while a configuration has been described here wherein the
optical beam of the third wavelength has the position of the third
emitting unit adjusted such that the optical beam of which the divergent
angle is converted by the collimator lens 42 and input to the diffraction
optical element 35 is in a diffused or converged state as to generally
parallel light, but a configuration may be made wherein the optical beam
is input to the diffraction optical element 35 by providing an element
which has wavelength selectivity and converts the divergent angle, or by
providing a mechanism which drives the collimator lens 42 in the optical
axis direction.

[0195]Also, while description has been made regarding a configuration
wherein the optical beams of the first and second wavelengths are input
to the diffraction optical element 35 in a state of generally parallel
light, while the optical beam of the third wavelength is input to the
diffraction optical element 35 in a diffused or converged state, the
present invention is not restricted to this arrangement, and
configurations may be made wherein, for example, all of the first through
third wavelength optical beams are input to the diffraction optical
element 35 in a state of generally parallel light, or wherein any or all
of the first through third wavelength optical beams are input to the
diffraction optical element 35 in a diffused or converged state.

[0196]The optical pickup 3 to which the present invention has been applied
has first through third emitting units for emitting optical beams of
first through third wavelengths, an object lens 34 for condensing the
optical beams of first through third wavelengths emitted from the first
through third emitting units into a signal recording face of an optical
disc, and a diffraction unit 50 provided on one face of an optical
element disposed on the outgoing optical path of the optical beams of
first through third wavelengths, wherein the diffraction unit 50 has
first through third diffraction regions 51, 52, and 53, with the first
through third diffraction regions 51, 52, and 53 being different
diffraction structures ring-shaped and having a predetermined depth, and
the first through third diffraction structures whereby optical beams of
each wavelength are diffracted such that diffracted light of a
predetermined diffraction order is dominant as described above, and
according to this configuration, optical beams corresponding to each of
three types of optical discs having difference usage wavelengths can be
appropriately condensed on the signal recording face using the shared
object lens 34, thereby realizing excellent recording and/or playing of
information signals to/from the respective optical discs by realizing
three-wavelength compatibility with the common object lens 34, without
necessitating a complex structure.

[0197]That is to say, the optical pickup 3 to which the present invention
has been applied obtains optimal diffraction efficiencies and diffraction
angels for the first through third wavelength optical beams due to the
diffraction unit 50 provided on one face within the optical path thereof,
whereby signals can be read from and written to the multiple types of
optical discs 11, 12, and 13, using the optical beams of different
wavelengths emitted from the multiple emitting units provided to each of
the light sources 31, 32, and 33, and also optical parts such as the
object lens 34 and so forth can be shared, thereby reducing the number of
parts, simplifying and reducing the size of the configuration, and
realizing high production and lower costs.

[0198]Also, the optical pickup 3 to which the present invention has been
applied can share the object lens 34 between the three wavelengths,
thereby preventing trouble of reduction of sensitivity of the actuator
due to increase weight of moving parts. Also, the optical pickup 3 to
which the present invention has been applied can sufficiently reduce
spherical aberration which is problematic in the case of sharing the
common object lens 34 between the three wavelengths, due to the
diffraction unit 50 provided on one face of the optical element, so
problems such as positioning of diffraction units in the event that
multiple diffraction units are provided on multiple faces to reduce
spherical aberration as with the related art, and deterioration of
diffraction efficiency due to providing of the multiple diffraction
units, can be prevented, which realizes simplification of the assembly
process and improved usage efficiency of light.

[0199]Further, the optical pickup 3 to which the present invention has
been applied not only realizes three-wavelength compatibility with the
diffraction unit 50 provided on the one face of the diffraction optical
element 35 described above, but also can perform aperture restriction
with a numerical aperture corresponding to each of the three types of
optical discs and three types of optical disc wavelengths, which enables
further simplification of configuration, reduction in size, and reduction
in costs.

[0200]Also, while the above optical pickup 3 has been described having the
first emitting unit provided at the first light source 31, the second
emitting unit provided at the second light source 32, and the third
emitting unit provided at the third light source 33, the present
invention is not restricted to this arrangement, and an arrangement may
be made wherein a light source having two of the first through third
emitting units, and another light source having the remaining one
emitting unit, are provided at different positions.

[0201]Next, description will be made regarding an optical pickup 60 shown
in FIG. 19 including a light source having a first emitting unit, and a
light source having second and third emitting units. Note that portions
in the following description which are the same as with the optical
pickup 3 will be denoted with the same reference numerals, and
description thereof will be omitted.

[0202]As shown in FIG. 19, the optical pickup 60 to which the present
invention has been applied includes a first light source 61 having a
first emitting unit for emitting an optical beam of a first wavelength, a
second light source 62 having a second emitting unit for emitting an
optical beam of a second wavelength and a third emitting unit for
emitting an optical beam of a third wavelength, an object lens 34 for
condensing optical beams emitted from the first through third emitting
units onto the signal recording face of an optical disc 2, and a
diffraction optical element 35 provided on the optical path between the
first through third emitting units and the object lens 34.

[0203]Also, the optical pickup 60 includes a beam splitter 63 serving as
an optical path synthesizing unit for synthesizing the optical paths of
the optical beam of the first wavelength that has been emitted from the
first emitting unit of the first light source 61 and the optical beams of
the second and third wavelengths that have been emitted from the second
and third emitting unit of the second light source 62, and a beam
splitter 64 serving the same function as the above third beam splitter
38.

[0204]Further, the optical pickup 60 has a first grating 39, and a grating
65 with wavelength dependency, provided between the second light source
62 and the beam splitter 63, for diffracting the optical beams of the
second and third wavelengths that have been emitted from the second and
third emitting units into three beams, for detection of tracking error
signals and so forth.

[0205]Also, the optical pickup 60 has a collimator lens 42, quarter-wave
plate 43, redirecting mirror 44, photosensor 45, and multi-lens 46, and
also a collimator lens driving unit 66 for driving the collimator lens 42
in the optical axis direction. The collimator lens driving unit 66 can
adjust the divergent angle of optical beams passing through the
collimator lens 42 as described above, whereby not only can spherical
aberration be reduced, but in the event that the mounted optical disc is
a so-called multi-layer optical disc having multiple signal recording
faces, recording and/or playing to/from each of the signal recording
faces is enabled by driving the collimator lens 42 in the optical axis
direction.

[0206]With the optical pickup 60 configured as described above, the
functions of each of the optical parts is the same as with the optical
pickup 3 except for those mentioned above, and the optical paths of the
optical beams of the first through third wavelengths emitted from the
first through third emitting units are the same as with the optical
pickup 3 except for the above-mentioned, i.e., following synthesizing of
the optical paths of the optical beams of each wavelength by the beam
splitter 64, so detailed description thereof will be omitted.

[0207]The optical pickup 60 to which the present invention has been
applied has first through third emitting units for emitting optical beams
of first through third wavelengths, an object lens 34 for condensing the
optical beams of first through third wavelengths emitted from the first
through third emitting units into a signal recording face of an optical
disc, and a diffraction unit 50 provided on one face of an optical
element disposed on the outgoing optical path of the optical beams of
first through third wavelengths, wherein the diffraction unit 50 has
first through third diffraction regions 51, 52, and 53, with the first
through third diffraction regions 51, 52, and 53 being different
diffraction structures ring-shaped and having a predetermined depth, and
the first through third diffraction structures whereby optical beams of
each wavelength are diffracted such that diffracted light of a
predetermined diffraction order is dominant as described above, and
according to this configuration, optical beams corresponding to each of
three types of optical discs having difference usage wavelengths can be
appropriately condensed on the signal recording face using the shared
object lens 34, thereby realizing excellent recording and/or playing of
information signals to/from the respective optical discs by realizing
three-wavelength compatibility with the common object lens 34, without
necessitating a complex structure. The optical pickup 60 also has the
other advantages of the above-described optical pickup 3, as well.

[0208]Further, the optical pickup 60 is configured such that the second
and third emitting units are positioned at a common light source 62,
thereby realizing further simplification of structure and reduction in
size. Note that in the same way, with the optical pickup to which the
present invention has been applied, the first through third emitting
units may be positioned at a common light source at generally the same
position, thereby realizing further simplification of structure and
reduction in size with such a configuration.

[0209]The optical disc device 1 to which the present invention has been
applied has a driving unit for holding and rotationally driving an
optical disc arbitrarily selected from the first through third optical
discs, and an optical pickup for performing recording and/or playing of
information signals from/to the optical disc being rotationally driven by
the driving unit by selectively irradiating one of multiple optical beams
of different wavelengths corresponding to the optical disc, and by using
the above-described optical pickups 3 or 60 as the optical pickup,
optical beams corresponding to each of three types of optical discs
having difference usage wavelengths can be appropriately condensed on the
signal recording face due to the diffraction unit provided on one face of
the optical element on the optical path of the optical beams of the first
through third wavelengths, using a common object lens 34, thereby
realizing excellent recording and/or playing by realizing
three-wavelength compatibility with the common object lens 34, while
enabling simplification of the structure and reduction in size, without
necessitating a complex structure.

<3>Second Embodiment of Optical Pickup (FIGS. 20 Through 36)

[0210]Next, an optical pickup 103 to which the present invention is
applied will be described as a second embodiment of the optical pickup
used in the above-described optical disc device 1, with reference to
FIGS. 20 through 36. As described above, the optical pickup 103 is an
optical pickup which selectively irradiates multiple optical beams with
different wavelengths onto optical discs arbitrarily selected from first
through third optical discs 11, 12, and 13, of which the format such as
the thickness of the protective layer differs, thereby performing
recording and/or playing of information signals.

[0211]As shown in FIG. 20, the optical pickup to which the present
invention has been applied includes a first light source 131 having a
first emitting unit for emitting an optical beam of a first wavelength, a
second light source 132 having a second emitting unit for emitting an
optical beam of a second wavelength longer than the first wavelength, a
third light source 133 having a third emitting unit for emitting an
optical beam of a third wavelength longer than the second wavelength, an
object lens 134 for condensing optical beams emitted from the emitting
unit of the first through third emitting units onto the signal recording
face of an optical disc 2, and a diffraction optical element 135 provided
on the optical path between the first through third emitting units and
the object lens 134.

[0212]Also, the optical pickup 103 includes a first beam splitter 136
provided between the second and third emitting units and the diffraction
optical element 135, serving as an optical path synthesizing unit for
synthesizing the optical paths of the optical beam of the second
wavelength that has been emitted from the second emitting unit and the
optical beam of the third wavelength that has been emitted from the third
emitting unit, a second beam splitter 137 provided between the first beam
splitter 136 and the diffraction optical element 135, serving as an
optical path synthesizing unit for synthesizing the optical path of the
optical beams of the second and third wavelengths of which the optical
paths have been synthesized by the first beam splitter 136 and the
optical beam of the first wavelength emitted form the frits emitting
unit, and a third beam splitter 138 provided between the second beam
splitter 137 and the diffraction optical element 135, serving as an
optical path splitting unit for splitting the outgoing optical path of
the optical beams of the first through third wavelengths synthesized at
the second beam splitter 137 from the returning optical path of the
optical beams of the first through third wavelengths reflected off of the
optical disc (hereinafter also referred to as "return path").

[0213]Further, the optical pickup 103 has a first grating 139 provided
between the first emitting unit of the first light source unit 131 and
the second beam splitter 137, for diffracting the optical beam of the
first wavelength that has been emitted from the first emitting unit into
three beams, for detection of tracking error signals and so forth, a
second grating 140 provided between the second emitting unit of the
second light source unit 132 and the first beam splitter 136, for
diffracting the optical beam of the second wavelength that has been
emitted from the second emitting unit into three beams, for detection of
tracking error signals and so forth, and a third grating 141 provided
between the third emitting unit of the third light source unit 133 and
the first beam splitter 136, for diffracting the optical beam of the
third wavelength that has been emitted from the third emitting unit into
three beams, for detection of tracking error signals and so forth.

[0214]Also, the optical pickup 103 has a collimator lens 142 provided
between the third beam splitter 138 and the diffraction optical element
135, serving as a divergent angle conversion unit for converting the
divergent angle of the optical beams of the first through third
wavelengths of which the optical paths have been synthesized at the third
beam splitter 138 so as to be adjusted into a state of generally parallel
light or a state diffused or converged as to generally parallel light,
and outputting, a quarter-wave plate 143 provided between the collimator
lens 142 and the diffraction optical element 135, so as to provide
quarter-wave phase difference to the optical beams of the first through
third wavelengths of which the divergent angle has been adjusted by the
collimator lens 142, and a redirecting mirror 144 provided between the
diffraction optical element 135 and the quarter-wave plate 143, for
redirecting by reflecting the optical beam which has passed through the
above-described optical parts within a plane generally orthogonal to the
optical axis of the object lens 134 and diffraction optical element 135,
so as to emit the optical beam in the optical axis direction toward the
object lens 134 and diffraction optical element 135.

[0215]Further, the optical pickup 103 includes a photosensor 145 for
receiving and detecting the optical beams of the first through third
wavelengths split at the third beam splitter 138 on the return path from
the optical beams of the first through third wavelengths on the outgoing
path, and a multi lens 146 provided between the third beam splitter 138
and the photosensor 145, for condensing optical beams of the first
through third wavelengths split at the third beam splitter 138 return
path onto the photoreception face of a photodetector or the like of the
photosensor 145, and also providing astigmatism for detecting focus error
signals or the like.

[0216]The first light source 131 has a first emitting unit for emitting an
optical beam of a first wavelength around 405 nm onto the first optical
disc 11. The second light source 132 has a second emitting unit for
emitting an optical beam of a second wavelength around 655 nm onto the
second optical disc 12. The third light source 133 has a third emitting
unit for emitting an optical beam of a third wavelength around 785 nm
onto the third optical disc 13. Note that while the first through third
emitting units are configured disposed at individual light sources 131,
132, and 133, the invention is not restricted to this, and an arrangement
may be made wherein two emitting units of the first through third
emitting units are disposed at one light source and the remaining
emitting unit is disposed at another light source at a different
position, or wherein the first through third emitting units are disposed
so as to form a light source at generally the same position.

[0217]The object lens 134 condenses the input optical beams of the first
through third wavelengths into the signal recording face of the optical
disc 2. The object lens 134 is movably held by an object lens driving
mechanism such as an unshown biaxial actuator or the like. The object
lens 134 is driven along two axes, one in the direction toward/away from
the optical disc 2, and the other in the radial direction of the optical
disc 2, by being moved by a biaxial actuator or the like based on the
tracking error signals and focus error signals generated from the RF
signals of the return light from the optical disc 2 that has been
detected at the photosensor 145. The object lens 134 condenses optical
beams emitted from the first through third emitting units such that the
optical beams are always focused on the signal recording face of optical
disc 2, and also causes the condensed optical beam to track a recording
track formed on the signal recording face of the optical disc 2. Note
that a configuration wherein the later-described diffraction optical
element 135 is held by a lens holder of the object lens driving mechanism
where the object lens 134 is held so as to be integral with the object
lens 134 enables the later-described advantages of a diffraction unit 150
provided to the diffraction optical element 135 to be suitably manifested
at the time of field shift of the object lens 134 such as movement in the
tracking direction.

[0218]The diffraction optical element 135 has, as one face thereof for
example, a diffraction unit 150 having multiple diffraction regions on
the incident side face thereof, with the diffraction unit 150 diffracting
each of the optical beams of the first through third wavelengths passing
through each of the multiple diffraction regions into predetermined
orders and inputting into the object lens 134, i.e., inputting into the
object lens 134 as optical beams in a diffused state or converged state
having a predetermined divergent angle, whereby the single object lens
134 can be used to perform suitable condensing of the optical beams of
the first through third wavelengths such that spherical aberration does
not occur at the signal recording face of the three types of optical
discs corresponding to the optical beams of the first through third
wavelengths. The diffraction optical element 135 serves as a condensation
optical device along with the object lens 134 to appropriately perform
condensation such that no spherical aberration occurs at the signal
recording face of the three types of optical discs corresponding to the
optical beams of the three different wavelengths.

[0219]The diffraction optical element 135 having the diffraction unit 150
performs diffraction of the first wavelength optical beam BB0 which has
transmitted the diffraction unit 150 so as to become +1st order
diffracted beam BB1 and inputs to the object lens 134, i.e., as an
optical beam in a diffused state having a predetermined divergent angle,
thereby appropriately condensing on the signal recording face of the
first optical disc 11, as shown in FIG. 21A, performs diffraction of the
second wavelength optical beam BD0 which has transmitted the diffraction
unit 150 so as to become -1st order diffracted beam BD1 and inputs to the
object lens 134, i.e., as an optical beam in a converged state having a
predetermined divergent angle, thereby appropriately condensing on the
signal recording face of the second optical disc 12, as shown in FIG.
21B, and performs diffraction of the third wavelength optical beam BC0
which has transmitted the diffraction unit 150 so as to become -2nd order
diffracted beam BC1 and inputs to the object lens 134, i.e., as an
optical beam in a converged state having a predetermined divergent angle,
thereby appropriately condensing on the signal recording face of the
third optical disc 13, as shown in FIG. 21C, for example, whereby
suitable condensation can be performed such that no spherical aberration
occurs at the signal recording face of the three types of optical discs,
with a single object lens 134. While description has been made here with
an example wherein optical beams of the same wavelength are made to be
diffracted beams of the same diffraction order at the multiple
diffraction regions of the diffraction unit 150, with reference to FIGS.
21A through 21C, the diffraction unit 150 configuring the optical pickup
103 to which the present invention is applied enables diffraction order
corresponding to each wavelength to be set for each region as described
later, so as to further reduce spherical aberration.

[0220]Note that in the above and following description of diffraction
orders, an order of diffraction which draws closer to the optical axis
side in the direction of progression with regard to an input optical beam
is a positive order. In other words, an order which diffracts toward the
optical axis direction of the input optical beam is a positive order.
That is to say, with the above first through third wavelengths, +1 order
diffracted light selected so as to be dominant diffracts in the direction
of convergence as compared to the input optical beams of each wavelength.

[0221]Specifically, as shown in FIG. 22, the diffraction unit 150 provided
at the incident side face of the diffraction optical element 135 has a
generally-circular first diffraction region 151 provided on the innermost
portion (hereinafter also referred to as "inner ring zone"), a
ring-shaped second diffraction region 152 provided on the outer side of
the first diffraction region 151 (hereinafter also referred to as "middle
ring zone"), and a ring-shaped third diffraction region 153 provided on
the outer side of the second diffraction region 152 (hereinafter also
referred to as "outer ring zone").

[0222]The first diffraction region 151 which is an inner ring zone has a
first diffraction structure formed having a ring shape with a
predetermined depth, and diffracts the optical beam of the first
wavelength that is transmitted therethrough such that diffracted light of
an order which forms an appropriate spot condensed on the signal
recording face of the first optical disc via the object lens 134 is
dominant, i.e., such that maximum diffraction efficiency is manifested
regarding diffracted light of other orders.

[0223]The first diffraction region 151 diffracts the optical beam of the
second wavelength that is transmitted therethrough such that diffracted
light of an order which forms an appropriate spot condensed on the signal
recording face of the second optical disc via the object lens 134 is
dominant, i.e., such that maximum diffraction efficiency is manifested
regarding diffracted light of other orders, by way of the first
diffraction structure.

[0224]The first diffraction region 151 diffracts the optical beam of the
third wavelength that is transmitted therethrough such that diffracted
light of an order which forms an appropriate spot condensed on the signal
recording face of the third optical disc via the object lens 134 is
dominant, i.e., such that maximum diffraction efficiency is manifested
regarding diffracted light of other orders, by way of the first
diffraction structure.

[0225]Thus, the first diffraction region 151 has a diffraction structure
suitably formed whereby diffracted light of a predetermined order is
dominant in the optical beam of each wavelength, thereby enabling
correction and reduction of spherical aberration at the time of optical
beams of each wavelength that have passed through the first diffraction
region 151 and become diffracted light of a predetermined order being
condensed on the signal recording face of the respective optical discs by
the object lens 134.

[0226]Specifically, as shown in FIGS. 22 and 23A, the first diffraction
region 151 is formed with the cross-sectional form of ring shapes
centered on the optical axis being formed in a blazed shape having a
predetermined depth (hereinafter also referred to as "groove depth") d.
Note that the cross-sectional form of the ring shapes in this diffraction
structure means the cross-sectional form of the rings taken along a plane
including the radial direction of the rings, i.e., a plane orthogonal to
the tangential direction of the rings. Also, in FIG. 23A, the saw-tooth
shape is formed such that the slopes of the protrusions and recesses heat
inward in the radial direction, which is to make the selected diffraction
order positive, and obtain a converged state with a desired divergent
angle. Note that here, a divergent angle for obtaining a converged state
is a negative divergent angle. The symbol Ro in FIGS. 23A through
23C represents the direction toward the outer side in the radial
direction of the rings, i.e., the direction away from the optical axis.

[0227]Note that in the first diffraction structure formed at the first
diffraction region 151, the groove width is determined taking into
consideration the dominant diffraction order and diffraction efficiency.
Also, as shown in FIG. 23A, the groove width is smaller in value the
farther away from the optical axis. Note that the groove widths are
determined based on phase difference obtained at the diffraction regions
formed with the groove widths, such that the spot condensed on the signal
recording face of the optical disc is optimal.

[0228]Also, in a case wherein the first diffraction region 151 diffracts
the optical beam of the first wavelength which is transmitted
therethrough such that diffracted light of the k1i'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, diffracts
the optical beam of the second wavelength which is transmitted
therethrough such that diffracted light of the k2i'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, and
diffracts the optical beam of the third wavelength which is transmitted
therethrough such that diffracted light of the k3i'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, k1i, k2i,
and k3i are such that (k1i, k2i, k3i)=(+1, +1, +1).

[0229]Now, as a first aspect regarding the first diffraction region 151,
there is need to reduce spherical aberration at each wavelength, as a
second aspect, there is the need to take into consideration
temperature-spherical aberration properties, i.e., there is the need to
reduce spherical aberration occurring during temperature change, and as a
third perspective, the structure must be advantageous in manufacturing,
and from these, the above diffraction orders k1i, k2i, and k3i have been
selected as diffraction orders with maximum diffraction efficiency, a
point which will be described below.

[0230]First, the first perspective will be described. Generally, in a
region having a function such as the first diffraction region 151, it is
known that satisfying the conditional expression

(λ1×k1x-k2×k2x)/(t1-t2)≈(λ1×k1x-k3.t-
imes.k3x)/(t1-t3) [0231]where λ1 is the first wavelength (nm),
λ2 is the second wavelength (nm), λ3 is the third wavelength
(nm), k1i is the diffraction order where an optical beam of the first
wavelength is selected, k2i is the diffraction order where an optical
beam of the second wavelength is selected, k3i is the diffraction order
where an optical beam of the third wavelength is selected, t1 is the
thickness (mm) of the first protective layer of the first optical disc,
t2 is the thickness (mm) of the second protective layer of the second
optical disc, t3 is the thickness (mm) of the third protective layer of
the third optical disc, and x=i for the inner ring zone in k1x, k2x, and
k3x in this conditional expression,

[0232]is a condition whereby spherical aberration on the signal recording
face of each optical disc at each wavelength can be corrected and
reduced. In the first diffraction region 151 which is the above-described
inner ring zone, when λ1=405 (nm), λ2=655 (nm), λ3=785
(nm), t1=0.1 (mm), t2=0.6 (mm), and t3=1.1 (mm), then k1i=+1, k2i=+1, and
k3i=+1, each hold, thereby satisfying the conditional expression, and it
has been confirmed that spherical aberration can be reduced. This can be
restated in other words that when plotting points Pλ1, Pλ2,
and Pλ3 in the graph in FIG. 24 wherein the horizontal axis
represents a value calculated by wavelength×diffraction order (nm)
and the vertical axis represents the thickness (mm) of the protective
layer, in the event of being situated on a straight line this means that
the spherical aberration on the signal recording face of each optical
disc of each wavelength can be corrected and reduced; in reality, in the
case of plotting the points Pλ1, Pλ2, and Pλ3 under the
conditions described below, the points are on a generally straight design
line, meaning that spherical aberration can be realized. Specifically,
the object lens 134 has the material of which it is configured, and the
face shape at the input and output sides, determined with the line L11 in
FIG. 24 as the design line, with the inclination of the design line
approximating the inclination of the line connecting Pλ1 and
Pλ2 calculated by (t1-t2)/(λ1×k1x-λ2×k2x)
or the inclination of the line connecting Pλ1 and Pλ3
calculated by (t1-t3)/(λ1×k1x-λ3×k3x), or
determined taking into consideration the inclination of these lines and
other design conditions. Note that while in FIG. 24 Pλ3 deviates
slightly upwards from the line, spherical aberration can be corrected in
a sure manner by inputting the incident light to the one of the object
lens 134 and diffraction optical element 135 which is closer to the
emitting units, which is the diffraction optical element 135 in this
case, as a divergent ray.

[0233]Next, the second perspective will be described. In a region having a
function such as the first diffraction region 151, these orders must be
positive in order to realize suitable temperature-spherical aberration
properties, i.e., reduction in spherical aberration without depending on
temperature change. Now, a positive diffraction order is an order of
diffraction which draws closer to the optical axis side in the direction
of progression with regard to an input optical beam. Spherical aberration
which occurs due to rise in temperature is represented as the sum of an
effect term ΔWn due to refractive index fluctuation of the material
configuring the object lens 134 under change in temperature, and an
effect term ΔWλ due to wavelength fluctuation of the incident
optical beam under change in temperature, i.e., by ΔW which is
obtained by the relational expression

ΔW=ΔWn+ΔWλ.

[0234]Of these, the sign of the latter effect term ΔWλ due to
wavelength fluctuation is governed by the diffraction direction due to
the diffraction unit 150. The object lens 134 provides positive power
(refractive power), so the refractive index drops as the temperature
rises, consequently acting in the direction such that the positive power
is weakened, and the effect term Wn due to refractive index fluctuation
is ΔWn<0. There is the need for the effect term ΔWλ
due to wavelength fluctuation to be such that ΔWλ>0 holds
in order to cancel out this effect term Wn, i.e., such that the positive
power is increased at the diffraction unit 150 under rising temperature.
Accordingly, it is advantageous form the perspective of
temperature-spherical aberration properties for the diffraction orders at
the diffraction unit 150 to be positive.

[0235]Now, the fact that the spherical aberration occurring due to
temperature rise can be cancelled out due to a configuration such as
described will be described in further detail with reference to the
longitudinal aberration drawing in FIGS. 25A through 25C. Prior to
description with reference to FIGS. 25A through 25C, longitudinal
aberration will be described with reference to FIGS. 26A and 26B. In
FIGS. 26A and 26B the x-axial direction represents the optical axis
direction, and the y-axial direction represents the image height, i.e.,
the height from the optical axis in the direction orthogonal to the
optical axis.

[0236]As shown in FIG. 26A, generally, optical beams passing through a
lens with no aberration are condensed on the same image plane regardless
of the incident position in the direction orthogonal to the optical axis
of the lens, i.e., condensed equally at the paraxial image point A0.

[0237]On the other hand, as shown in FIG. 26B for example, optical beams
passing through a lens with aberration are condensed on different image
planes according to the incident position in the direction orthogonal to
the optical axis of the lens, i.e., condensed at positions shifted in the
x-axial direction from the paraxial image point B0. At this time, the
line LB indicating the state of longitudinal aberration is indicated by a
curve obtained by connecting points B1 through B7 for example, with the
height of the incident position of the optical beam from the optical axis
(image height) as the y-axis, and the position where the image plane of
rays input at the position this height from the optical axis and the
optical axis which is the principal ray intersect as the x-axis.
Specifically, the ray input at the height position y1 from the optical
axis intersects with the optical axis at the position x1, so a B1 at the
coordinates (x1, y1) is obtained. Also, the ray input at the height
position y2 from the optical axis intersects with the optical axis at the
position x2, so a B2 at the coordinates (x2, y2) is obtained. This holds
true for B3 through B7 as well, so detailed description will be omitted
here.

[0238]In the same way, with the lens shown in FIG. 26A, in the same way as
with the above-described line LB, the line LA indicating the state of
longitudinal aberration is indicated by a line obtained by connecting
points A1 through A7 for example, with the height of the incident
position of the optical beam from the optical axis as the y-axis, and the
position where rays input at the position this height from the optical
axis, and the optical axis, intersect as the x-axis. In the case in FIG.
26A, the position of the x-axis intersecting with the optical axis is
always constant regardless of the position on the y-axis, so the line LA
indicating the state of longitudinal aberration agrees with the y-axis.
Generally, a line indicating the state of longitudinal aberration can be
said to be representing a state of little or not aberration of in a state
of matching the y-axis as shown in FIG. 26A or in a state as close
thereto as possible.

[0239]Next, in light of the above, the fact that the spherical aberration
occurring due to rise in temperature can be cancelled out by selecting
the above-described diffraction orders k1i, k2i, and k3i will be
described with reference to FIGS. 25A through 25C.

[0240]FIGS. 25A and 25B are conceptual diagrams illustrating the effect
term ΔWn due to refractive power fluctuation of the composition
material under change in temperature, and the effect term ΔWλ
due to wavelength fluctuation of the incident optical beam under change
in temperature, as longitudinal aberration respectively. In FIGS. 25A and
25B, the dotted line Lwn represents the longitudinal aberration due to
refractive power fluctuation, i.e., represents the effect term ΔWn
due to refractive power fluctuation of the composition material under
change in temperature as longitudinal aberration, the single-dot broken
line Lwλ1 represents longitudinal aberration due to change in
diffraction angle in the event that the selected diffraction order is a
positive diffraction order which is to say positive refractive power is
provided by the diffraction unit, i.e., the effect term ΔWλ
due to wavelength fluctuation as longitudinal aberration, and the
single-dot broken line Lwλ2 represents longitudinal aberration in
the event that the selected diffraction order is a negative diffraction
order which is to say negative refractive power is provided by the
diffraction unit, i.e., the effect term ΔWλ due to wavelength
fluctuation as longitudinal aberration, for comparison with Lwλ1.
In FIGS. 25A and 25B, the solid lines Lw1 and Lw2 represent spherical
aberration ΔW occurring due to temperature rise, obtained by adding
the ΔWn and ΔWλ in FIG. 25A, as longitudinal
aberration. In FIG. 25B, the solid line Lw1 illustrates addition of the
dotted line Lwn in FIG. 25B and the single-dot broken line Lwλ1,
i.e., the spherical aberration ΔW in the event that the diffraction
order is positive, and the solid line Lw2 In FIG. 25A illustrates
addition of the dotted line Lwn in FIG. 25A and the single-dot broken
line Lwλ2, i.e., the spherical aberration ΔW in the event
that the diffraction order is negative.

[0241]As shown in FIG. 25B, in a region having a function such as the
first diffraction region 151, selecting the above-described diffraction
orders k1i, k2i, and k3i, i.e., selecting positive diffraction orders
enables a situation wherein aberration is suppressed, with the
longitudinal aberration state (Lw1) being close to the state in FIG. 26A.
Conversely, in the event that negative diffraction orders are selected as
shown in FIG. 25A, the state of longitudinal aberration (Lw2) does not
have aberration suppressed. That is to say, this is a problematic state
from the perspective of temperature-spherical aberration properties. As
described above, selecting the above-described diffraction orders k1i,
k2i, and k3i is advantageous from the perspective of
temperature-spherical aberration properties.

[0242]Next, the third perspective will be described. A diffraction unit
having a function such as the first diffraction region 151 is configured
with one face of the diffraction optical element 135, or one face of the
object lens as described later, having a diffraction structure formed
thereupon, so in the event that the diffraction order selected is very
great, the depth d of the diffraction structure to be formed becomes
deep. A deep diffraction structure depth d may not only lead to poor
formation precision; the optical path length increasing effect due to
temperature change is greater, and there may be a problem wherein the
temperature-diffraction efficiency property deteriorates. Due to such
reasons, diffractions orders up to around the 3rd order or 4th order is
suitable, and generally used. That is to say, the diffraction orders k1i,
k2i, and k3i, to be selected for the first diffraction region 151 are
such as described above, so from the perspective of manufacturing as
well, manufacturing is easy, there is no problem with deterioration in
precision or the like, quality can be improved, and consequently,
diffracted light having excellent diffraction efficiency can be emitted
in a sure manner.

[0243]Thus, the first diffraction region 151 serving as the inner ring
zone has selected excellent orders from the first perspective of
reduction of spherical aberration, the second perspective of
temperature-spherical aberration properties, and the third perspective
from depth of the diffraction structure formed in manufacturing, and
accordingly, the above configuration yields a configuration wherein
spherical aberration can be reduced, occurrence of aberration under
temperature change can be reduced, and which is advantageous in
manufacturing.

[0244]The second diffraction region 152 which is a middle ring zone has a
second diffraction structure formed which is ring shaped and has a
predetermined depth, and which is a different structure from the first
diffraction structure. The second diffraction region 152 diffracts the
optical beam of the first wavelength that is transmitted therethrough
such that diffracted light of an order which forms an appropriate spot
condensed on the signal recording face of the first optical disc via the
object lens 134 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders.

[0245]The second diffraction region 152 diffracts the optical beam of the
second wavelength that is transmitted therethrough such that diffracted
light of an order which forms an appropriate spot condensed on the signal
recording face of the second optical disc via the object lens 34 is
dominant, i.e., such that maximum diffraction efficiency is manifested
regarding diffracted light of other orders, by way of the second
diffraction structure.

[0246]The second diffraction region 152 diffracts the optical beam of the
third wavelength that is transmitted therethrough such that diffracted
light of orders other than an order which forms an appropriate spot
condensed on the signal recording face of the third optical disc via the
object lens 134 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders, by
way of the second diffraction structure. Note that the second diffraction
region 152 can sufficiently reduce diffraction efficiency diffracted
light of an order which forms an appropriate spot condensed on the signal
recording face of the third optical disc via the object lens 134 for the
optical beam of the third wavelength that is transmitted therethrough, by
way of the second diffraction structure.

[0247]Thus, the second diffraction region 152 has a diffraction structure
formed suitably whereby diffracted light of a predetermined order is
dominant in the optical beam of each wavelength, thereby enabling
correction and reduction of spherical aberration at the time of optical
beams of first and second wavelengths that have passed through the second
diffraction region 152 and become diffracted light of a predetermined
order being condensed on the signal recording face of the respective
optical discs by the object lens 134.

[0248]Also, the second diffraction region 152 is configured so as to
function as described above regarding the optical beams of the first and
second wavelengths, but such that diffracted light of orders other than
diffracted light of an order which is condensed on the signal recording
face of the third optical disc after passing through the second
diffraction region 152 and the object lens 134 is dominant, whereby
aperture restriction can be applied to the optical beam of the third
wavelength, such that even if the optical beam of the third wavelength
which has been transmitted through the second diffraction region 152 is
input to the object lens 134, there is very little effect on the signal
recording face of the third optical disc, i.e., markedly reducing the
light quantity of the optical beam of the third wavelength which is
condensed on the signal recording face of the third optical disc after
passing through the second diffraction region 152 and the object lens
134, to around zero.

[0249]Now, the above-described first diffraction region 151 is formed of a
size such that the optical beam of the third wavelength which has been
transmitted through the region thereof is input to the object lens 134 in
the same state as an optical beam which has been subjected to aperture
restriction at around NA=0.45, and since the second diffraction region
152 formed on the outer side of the first diffraction region 151 does not
allow condensation of the optical beam of the third wavelength which has
been transmitted through this region on the third optical disc via the
object lens 134, the diffraction unit 150 which has the first and second
diffraction regions 151 and 152 configured thus functions so as to
restrict the numerical aperture of the optical beam of the third
wavelength to around NA=0.45. It should be noted however, that while in
this arrangement of the diffraction unit 150, the optical beam of the
third wavelength is subjected to aperture restriction around NA=0.45, but
numerical aperture restriction due to the above configuration is not
limited to this.

[0250]Specifically, as shown in FIGS. 22 and 23A, in the same way as with
the above-described first diffraction region 151, the second diffraction
region 152 is formed with the cross-sectional form of ring shapes
centered on the optical axis being formed in a blazed shape having a
predetermined depth d.

[0251]Also, while description is made here with regard to the second
diffraction region 152 having the cross-sectional form of the rings
formed as a diffraction structure with a blazed form, any diffraction
structure may be used as long as an optical beam of a predetermined order
is dominant as to the optical beam of each wavelength as described above,
so a configuration may be used such as shown in FIG. 23B, with a
diffraction region 152B having a diffraction structure wherein the
cross-sectional form of the rings is formed with the cross-sectional form
of ring shapes centered on the optical axis being formed in a
staircase-like shape having a predetermined depth d and a predetermined
number of steps S, continuing in the radial direction in a staircase
form, for example.

[0252]Now, the diffraction structure having a staircase-like shape with a
predetermined number of steps S is a structure wherein a staircase-like
shape having first through S'th steps of which the depths are
approximately the same is configured continuing in the radial direction,
and further, in other words, is a structure having first through S+1'th
diffraction faces formed at approximately the same interval in the
optical axis direction. Also, the predetermined depth d in the
diffraction structure means the length along the optical axis direction
between the diffraction face of the S+1'th diffraction face which is
formed at the side of the staircase form closest to the surface (i.e.,
the highest step, which is the shallowest position) and diffraction face
of the first diffraction face which is formed at the side of the
staircase form closest to the element (i.e., the lowest step, which is
the deepest position). This holds true for later-described FIG. 23C as
well. Note that while a structure has been illustrated in FIGS. 23B and
23C wherein the steps of each stepped portion of the staircase shape are
formed such that the closer to the inner side in the radial direction,
the closer to the surface side the steps are formed, an arrangement which
has been made to select positive diffraction orders, and to obtain a
convergent state with a desired divergent angle. In the second and the
later-described third diffraction structures, the groove depth d and
number of steps S in the case of having a staircase form are determined
taking into consideration the dominant diffraction order and diffraction
efficiency.

[0253]Also, as shown in FIGS. 23B and 23C, the groove width of each step
(the radial-direction dimension of each step portion of the staircase
form) is such that the steps are formed with equal width within one
staircase form, while looking at the different staircase forms formed
continuously in the radial direction, the value of the step width is
greater at staircase forms further away form the optical axis. Note that
the groove widths are determined based on phase difference obtained at
the diffraction regions formed with the groove widths, such that the spot
condensed on the signal recording face of the optical disc is minimal.

[0254]For example, the diffraction structure of the second diffraction
region 152B is, as shown in FIG. 23B, a diffraction structure having a
staircase portion including first through third steps 152s1, 152s2, and
152s3, formed continuously in the radial direction, wherein the number of
steps is 3 (S=3), and the depth of each step is generally the same depth
(d/3), and first through fourth diffraction faces 152f1, 152f2, 152f3,
and 152f4 formed at the same intervals of d/3 in the optical axis
direction.

[0255]Also, in a case wherein the second diffraction region 152 diffracts
the optical beam of the first wavelength which is transmitted
therethrough such that diffracted light of the k1m'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, and
diffracts the optical beam of the second wavelength which is transmitted
therethrough such that diffracted light of the k2m'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, the
diffraction orders k1m and k2m are in the following relation.

(k1m, k2m)=(+1, +1), (+3, +2).

[0256]Now, the second diffraction region 152 serving as the inner ring
zone is obtained by orders most excellent from the first through third
perspectives described in the above description of the first diffraction
region 151, and accordingly, spherical aberration can be reduced,
occurrence of aberration under temperature change can be reduced, a
configuration which is advantageous in manufacturing can be had.

[0257]Now, as described above, the second diffraction region 152 is
configured so as to diffract light such that the diffraction efficiency
of the diffracted light of the diffraction orders k1m and k2m for the
optical beams of the first and second wavelengths passing through the
object lens 134 is in a high state, so as to form a suitable spot
condensed on the signal recording faces of the first and second optical
discs, and also to have an aperture restriction function for suppressing
the diffraction efficiency of the diffraction order of the optical beam
of the third wavelength to be condensed on the signal recording face of
the third optical disc as much as possible, but a configuration may be
made wherein the optical beam of this diffraction order in the optical
beam of the third wavelength is shifted from a state wherein the focal
point is imaged on the signal recording face of the third optical disc,
so as to further reduce the light quantity of the optical beam
substantially condensed on the third optical disc. Note that hereinafter,
shifting the position where the optical beam of a predetermined
wavelength is imaged via the object lens 34, from the signal recording
face of the corresponding optical disc, so as to substantially reduce the
light quantity of the optical beam condensed on the signal recording
face, will be also referred to as "flaring".

[0258]Now, with regard to the second diffraction region 152, flaring and
the configuration thereof will be described. Description has been made
above regarding the first diffraction region 151 that there is the need
to satisfy the conditional expression of

(λ1×k1x-λ2×k2x)/(t1-t2)≈(λ1×k1-
x-λ3×k3x)/(t1-t2),

[0259]this conditional expression (x=m for the middle ring zone in k1x,
k2x, and k3x in this conditional expression) being taken into
consideration in the second diffraction region 152 as well. With this
second diffraction region 152 serving as the middle ring zone, giving
thought to the function of diffracting light such that the diffraction
efficiency of the diffracted light of the diffraction orders k1m and k2m
for the optical beams of the first and second wavelengths passing through
the object lens 134 is in a high state, so as to form a suitable spot on
the signal recording faces of the first and second optical discs, as
described above, the Pλ1 and Pλ2 to be plotted can be
positioned on a design line, and further, a design line can be selected
such that the Pλ3 intentionally deviates from the design line so as
to cause flaring regarding the third wavelength. That is to say,
configuring the object lens 134 formed based on a design line wherein
Pλ3 deviates from the design line allows the diffracted rays of the
diffraction order of the optical beam of the third wavelength to be
shifted from a state of imaging the focal point on the signal recording
face of the third optical disc, so the quantity of light of the optical
beam of the third wavelength condensed on the signal recording face of
the third optical disc can be substantially reduced, and accordingly,
aperture restriction regarding the optical beam of the third wavelength
as described above can be performed in a sure an excellent manner.
Specifically, in the event that (k1m, k2m, k3m)=(+3, +2, +2) as described
later with reference to FIG. 33, Pλ3 deviates from the design line
L13, so in addition to the effect of reducing the diffraction efficiency
of the order of the third wavelength, due to the diffraction structure
formed on the second diffraction region 152 which is an initially
expected advantage, the advantages of flaring can also be obtained,
thereby enabling the quantity of light of the optical beam of the third
wavelength input to the third optical disc to be further suppressed.

[0260]The third diffraction region 153 which is an outer ring zone has a
third diffraction structure formed which is ring shaped and has a
predetermined depth, and which is a different structure from the first
and second diffraction structures. The third diffraction region 153
diffracts the optical beam of the first wavelength that is transmitted
therethrough such that diffracted light of an order which forms an
appropriate spot condensed on the signal recording face of the first
optical disc via the object lens 134 is dominant, i.e., such that maximum
diffraction efficiency is manifested regarding diffracted light of other
orders.

[0261]Also, the third diffraction region 153 diffracts the optical beam of
the second wavelength that is transmitted therethrough such that
diffracted light of orders other than an order which forms an appropriate
spot condensed on the signal recording face of the second optical disc
via the object lens 134 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders, by
way of the third diffraction structure. Note that the third diffraction
region 153 diffracts the optical beam of the second wavelength that is
transmitted therethrough such that diffraction efficiency of diffracted
light of an order which forms an appropriate spot condensed on the signal
recording face of the second optical disc via the object lens 134 is
sufficiently reduced, by way of the third diffraction structure.

[0262]Also, the third diffraction region 153 diffracts the optical beam of
the third wavelength that is transmitted therethrough such that
diffracted light of orders other than an order which forms an appropriate
spot condensed on the signal recording face of the third optical disc via
the object lens 134 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders, by
way of the third diffraction structure. Note that the third diffraction
region 153 can sufficiently reduce diffraction efficiency diffracted
light of an order which forms an appropriate spot on the signal recording
face of the third optical disc via the object lens 134 for the optical
beam of the third wavelength that is transmitted therethrough, by way of
the third diffraction structure.

[0263]Thus, the third diffraction region 153 has a diffraction structure
suitably formed whereby diffracted light of a predetermined order is
dominant in the optical beam of each wavelength, thereby enabling
correction and reduction of spherical aberration at the time of the
optical beam of the first wavelength that has passed through the third
diffraction region 153 and become diffracted light of a predetermined
order being condensed on the signal recording face of the optical disc by
the object lens 134.

[0264]Also, the third diffraction region 153 is configured so as to
function as described above regarding the optical beams of the first
wavelength, but such regarding optical beams of the second and third
wavelength that diffracted light of orders other than diffracted light of
an order which is condensed on the signal recording face of the second
and third optical discs after passing through the third diffraction
region 153 and the object lens 134 is dominant, whereby aperture
restriction can be applied to the optical beam of the second wavelength,
such that even if the optical beam of the second and third wavelengths
which have been transmitted through the third diffraction region 153 is
input to the object lens 134, there is very little effect on the signal
recording face of the third optical disc, i.e., markedly reducing the
light quantity of the optical beam of the third wavelength which is
condensed on the signal recording face of the second and third optical
discs after passing through the third diffraction region 153 and the
object lens 134, to around zero. Note that the third diffraction region
153 can function so as to perform aperture restriction for the optical
beam of the third wavelength, along with the above-described second
diffraction region 152.

[0265]Now, the above-described second diffraction region 152 is formed of
a size such that the optical beam of the second wavelength which has been
transmitted through the region thereof is input to the object lens 134 in
the same state as an optical beam which has been subjected to aperture
restriction at around NA=0.6, and since the third diffraction region 153
formed on the outer side of the second diffraction region 152 does not
allow condensation of the optical beam of the second wavelength which has
been transmitted through this region on the optical disc via the object
lens, the diffraction unit 150 which has the second and third diffraction
regions 152 and 153 configured thus functions so as to restrict the
numerical aperture of the optical beam of the second wavelength to around
NA=0.6. It should be noted however, that while in this arrangement of the
diffraction unit 150, the optical beam of the second wavelength is
subjected to aperture restriction around NA=0.6, numerical aperture
restriction due to the above configuration is not limited to this.

[0266]Also, the third diffraction region 153 is formed of a size such that
the optical beam of the first wavelength which has been transmitted
through the region thereof is input to the object lens 134 in the same
state as an optical beam which has been subjected to aperture restriction
at around NA=0.85, and since there is no diffraction region formed on the
outer side of the third diffraction region 153, this does not allow
condensation of the optical beam of the first wavelength which has been
transmitted through this region on the first optical disc via the object
lens, and the diffraction unit 150 which has the third diffraction region
153 configured thus functions so as to restrict the numerical aperture of
the optical beam of the first wavelength to around NA=0.85. Note that
with the first wavelength optical beam transmitted through the third
diffraction region 153, light of diffraction of +1 order, +2 order, +3
order, +4 order, and +5 order, for example is dominant, so the zero-order
light transmitted through the region outside the third diffraction region
153 almost never passes through the object lens 134 to be condensed on
the first optical disc, but in cases wherein this zero-order light does
pass through the object lens 134 and is condensed on the first optical
disc, a configuration may be provided to perform aperture restriction by
providing, at the region outside of the third diffraction region 153,
either a shielding portion for shielding optical beams passing through,
or a diffraction region having a diffraction structure wherein optical
beams of orders other than the order of the optical beam passing through
the object lens 134 to be condensed on the first optical disc are
dominant. It should be noted however, that while in this arrangement of
the diffraction unit 150, the optical beam of the first wavelength is
subjected to aperture restriction around NA=0.85, but the present
invention is not restricted to this, i.e., numerical aperture restriction
due to the above configuration is not limited to this.

[0267]Specifically, as shown in FIG. 22 and 23A, in the same way as with
the above-described first diffraction region 151, the third diffraction
region 153 is formed with the cross-sectional form of ring shapes
centered on the optical axis being formed in a blazed shape having a
predetermined depth d, for example.

[0268]Also, while description is made here with regard to the second
diffraction region having the cross-sectional form of the rings formed as
a diffraction structure with a blazed form, any diffraction structure may
be used as long as an optical beam of a predetermined order is dominant
as to the optical beam of each wavelength as described above, so a
configuration may be used such as shown in FIG. 23C, with a diffraction
region 153B having a diffraction structure wherein the cross-sectional
form of the rings is formed with the cross-sectional form of ring shapes
centered on the optical axis being formed in a staircase-like shape
having a predetermined depth d and a predetermined number of steps S,
continuing in the radial direction in a staircase form, for example.

[0269]For example, the diffraction structure of the third diffraction
region 153B is, as shown in FIG. 23C, a diffraction structure having a
staircase portion including first and second steps 153s1 and 153s2,
formed continuously in the radial direction, wherein the number of steps
is 2 (S=2), and the depth of each step is generally the same depth (d/2),
and first through third diffraction faces 153f1, 153f2, and 153f3 formed
at the same intervals of d/2 in the optical axis direction.

[0270]Also, the third diffraction region 153 is configured such that the
diffraction order k1o is expressed with the following relation in a case
wherein the k1o order diffracted light of the optical beam of the first
wavelength transmitted therethrough is dominant, i.e., so that the
diffraction efficiency is maximum,

1≦k1o≦5

[0271]where k1o is a positive integer. That is to say, k1o is one of
k1o=+1, +2, +3, +4, or +5.

[0272]Now, the third diffraction region 153 serving as the outer ring zone
is selected by orders most excellent from the first through third
perspectives described in the above description of the first diffraction
region 151, and accordingly, spherical aberration can be reduced,
occurrence of aberration under temperature change can be reduced, a
configuration which is advantageous in manufacturing can be had.

[0273]Now, as described above, the third diffraction region 153 is
configured so as to diffract light such that the diffraction efficiency
of the diffracted light of the diffraction order k1o for the optical beam
of the first wavelength passing through the object lens 134 is in a high
state, so as to form a suitable spot condensed on the signal recording
faces of the first optical disc, and also to have an aperture restriction
function for suppressing the diffraction efficiency of the diffraction
order of the optical beam of the second and third wavelengths to be
condensed on the signal recording face of the second and third optical
discs as much as possible, but a configuration may be made wherein the
optical beam of this diffraction order in the optical beam of the second
and third wavelengths are shifted from a state wherein the focal point is
imaged on the signal recording face of the second and third optical
discs, so as to further reduce the light quantity of the optical beam
substantially condensed on the signal recording face of the second and
third optical discs, i.e., where flaring is employed.

[0274]Now, with regard to the third diffraction region 153, flaring and
the configuration thereof will be described. Description has been made
above regarding the first diffraction region 151 that there is the need
to satisfy the conditional expression of

(λ1×k1x-λ2×k2x)/(t1-t2)≈(λ1×k1-
x-λ3×k3x)/(t1-t3),

[0275]this conditional expression (x=o for the outer ring zone in k1x,
k2x, and k3x in this conditional expression) being taken into
consideration in the third diffraction region 153 as well. With regard to
the third diffraction region 153 serving as the outer ring zone, giving
thought to the function of diffracting light such that the diffraction
efficiency of the diffracted light of the diffraction order k1o for the
optical beams of the first wavelength passing through the object lens 134
is in a high state, so as to form a suitable spot condensed on the signal
recording faces of the first optical disc, as described above, the
Pλ1 to be plotted can be positioned on a design line, and further,
a design line can be selected such that Pλ2 and Pλ3
corresponding to the second and third wavelengths intentionally deviate
from the design line, so as to cause flaring regarding the second
wavelength or the third wavelength, or the second and third wavelengths.

[0276]That is to say, configuring the object lens 134 formed based on a
design line wherein Pλ2 deviates from the design line allows the
diffracted rays of the diffraction order of the optical beam of the
second wavelength to be shifted from a state of imaging the focal point
on the signal recording face of the second optical disc, so the quantity
of light of the optical beam of the second wavelength condensed on the
signal recording face of the second optical disc can be substantially
reduced, and accordingly, aperture restriction regarding the optical beam
of the second wavelength as described above can be performed in a sure an
excellent manner. Also, configuring the object lens 134 formed based on a
design line wherein Pλ3 deviates from the design line allows the
diffracted rays of the diffraction order of the optical beam of the third
wavelength to be shifted from a state of imaging the focal point on the
signal recording face of the third optical disc, so the quantity of light
of the optical beam of the third wavelength condensed on the signal
recording face of the third optical disc can be substantially reduced,
and accordingly, aperture restriction regarding the optical beam of the
third wavelength as described above can be performed in a sure an
excellent manner. Also, configuring the object lens 134 formed based on a
design line wherein both Pλ2 and Pλ3 deviate from the design
line allows both of the above-described advantages to be had, i.e., the
quantity of light of the optical beams of the second and third
wavelengths condensed on the signal recording face of the corresponding
optical discs can be reduced.

[0277]Specifically, in the event that (k1o, k2o, k3o)=(+1, +2, +2) as
described later with reference to FIG. 30, Pλ2 deviates from the
design line L12, so in addition to the effect of reducing the diffraction
efficiency of the diffracted light of the order of the second wavelength
due to the diffraction structure formed on the third diffraction region
153 which is an initially expected advantage, the advantages of flaring
can also be obtained, thereby enabling the quantity of light of the
optical beam of the second wavelength input to the second optical disc to
be further suppressed. Also, as described later with reference to FIG.
34, in the case of (k1o, k2o, k3o)=(+4, +3, +3) both Pλ2 and
Pλ3 deviate from the design line L14, so in addition to the effect
of reducing the diffraction efficiency of the diffracted light of the
orders of the second and third wavelengths due to the diffraction
structure formed on the third diffraction region 153 which is an
initially expected advantage, the advantages of flaring can also be
obtained, thereby enabling the quantity of light of the optical beam of
the second and third wavelengths input to the second and third optical
discs to be further suppressed.

[0278]Specific examples of the above-described diffraction unit 150 having
the first diffraction region 151 which is the inner ring zone, second
diffraction region 152 which is the middle ring zone, and third
diffraction region 153 which is the outer ring zone, will be given below,
with specific numerical values of the depth d and number of steps S in
the staircase form or blazing, and the diffraction order of diffracted
light of the order that is dominant in the optical beam of each
wavelength, and the diffraction efficiency of the diffracted light of
each diffraction order is shown in Table 4 and the later-described Table
5. Note that Table 4 illustrates a first embodiment of the diffraction
unit 150 and Table 5 illustrates a second embodiment of the diffraction
unit 150, wherein k1 in Tables 4 and 5 indicates the diffraction orders
(k1i, k1m, k1o) where the optical beam of the first wavelength is
condensed at each ring zone via the object lens 134 so as to form a
suitable spot condensed on the signal recording face of the first optical
disc, i.e., diffraction orders where diffraction efficiency is maximum,
eff1 illustrates the diffraction efficiency of the diffraction orders
(k1i, k1m, k1o) for the optical beam of the first wavelength, k2
indicates the diffraction orders (k2i, k2m, k2o) where the optical beam
of the second wavelength is condensed via the object lens 134 so as to
form a suitable spot on the signal recording face of the second optical
disc, i.e., diffraction orders where diffraction efficiency is maximum,
particularly at the inner ring zone and middle ring zone, eff2
illustrates the diffraction efficiency of the diffraction orders (k2i,
k2m, k2o) for the optical beam of the second wavelength, k3 indicates the
diffraction orders (k3i, k3m, k3o) where the optical beam of the third
wavelength is condensed via the object lens 134 so as to form a suitable
spot on the signal recording face of the third optical disc, i.e.,
diffraction orders where diffraction efficiency is maximum, particularly
at the inner ring zone, eff3 illustrates the diffraction efficiency of
the diffraction orders (k3i, k3m, k3o) for the optical beam of the third
wavelength, d indicates the groove depth of each diffraction region, and
S indicates the number of steps in the case of the staircase form, with
"∞" indicating a blazed shape. Note that the asterisks in Table 4
and Table 5 indicate diffraction order for condensing an optical beam
passing through the middle ring zone or the outer ring zone in each
embodiment so as to appropriately form a spot on the signal recording
face of the corresponding optical disk via the object lens 134, i.e., a
diffraction order whereby spherical aberration on the signal recording
face of the corresponding optical disc can be corrected, or a diffraction
order for a flared state as described later, and "≈0" indicates
that the diffraction efficiency is at a state of approximately zero.

[0279]Now, the first embodiment shown in Table 4 will be described. At the
inner ring zone in the first embodiment, as shown in Table 4, with a
blazed form (S=∞) having a groove depth of d=0.9 (μm), the
diffraction efficiency eff1=0.91 for the diffraction order k1i=+1 of the
optical beam of the first wavelength, the diffraction efficiency
eff2=0.73 for the diffraction order k2i=+1 of the optical beam of the
second wavelength, and the diffraction efficiency eff3=0.53 for the
diffraction order k3i=+1 of the optical beam of the third wavelength.

[0280]Next, the inner ring zone of the first embodiment will be described
in further detail with reference to FIGS. 27A through 27C. FIG. 27A is a
diagram illustrating change in the diffraction efficiency of the +1 order
diffracted light of the optical beam of the first wavelength in a case of
changing the groove depth d of the blazed form where the number of steps
S=∞, FIG. 27B is a diagram illustrating change in the diffraction
efficiency of the +1 order diffracted light of the optical beam of the
second wavelength in a case of changing the groove depth d of the blazed
form where the number of steps S=∞, and FIG. 27C is a diagram
illustrating change in the diffraction efficiency of the +1 order
diffracted light of the optical beam of the third wavelength in a case of
changing the groove depth d of the blazed form where the number of steps
S=∞. In FIGS. 27A through 27C, the horizontal axis represents the
groove depth in nm, and the vertical axis represents the diffraction
efficiency (intensity of light). As shown in FIG. 27A, at the position of
900 nm on the horizontal axis, eff1 is 0.91, eff2 is 0.73 as shown in
FIG. 27B, and eff3 is 0.53 as shown in FIG. 27C.

[0281]At the middle ring zone in the first embodiment, as shown in Table
4, with groove depth d=5.1 (μm) and the number of steps S=3, the
diffraction efficiency eff1=0.72 for the diffraction order k1m=+1 of the
optical beam of the first wavelength, and the diffraction efficiency eff2
=0.66 for the diffraction order k2m=+1 of the optical beam of the second
wavelength. Also, the diffraction efficiency eff3 for the diffraction
order k3m (*) of the optical beam of the third wavelength passing through
the region, for condensing light so as to form a spot with the optical
beam of the third wavelength on the signal recording face of the third
optical disc via the object lens 134 is approximately zero.

[0282]Next, the middle ring zone of the first embodiment will be described
in further detail with reference to FIGS. 28A through 28C. FIG. 28A is a
diagram illustrating change in the diffraction efficiency of the +1 order
diffracted light of the optical beam of the first wavelength in a case of
changing the groove depth d of the staircase form where the number of
steps S=3, FIG. 28B is a diagram illustrating change in the diffraction
efficiency of the +1 order diffracted light of the optical beam of the
second wavelength in a case of changing the groove depth d of the
staircase form where the number of steps S=3, and FIG. 28C is a diagram
illustrating change in the diffraction efficiency of the +1 order
diffracted light of the optical beam of the third wavelength in a case of
changing the groove depth d of the staircase form where the number of
steps S=3. In FIGS. 28A through 28C, the horizontal axis represents the
groove depth in nm, and the vertical axis represents the diffraction
efficiency (intensity of light). As shown in FIG. 28A, at the position of
5100 nm on the horizontal axis, eff1 is 0.72, eff2 is 0.66 as shown in
FIG. 28B, and eff3 is approximately zero as shown in FIG. 28C. Note that
in Table 4 and the above, the diffraction order of the optical beam of
the third wavelength noted with the asterisk "*" is +1.

[0283]Also, at the outer ring zone in the first embodiment, as shown in
Table 4, with a blazed form (S=∞) having a groove depth of d=0.6
(μm), the diffraction efficiency eff1=0.92 for the diffraction order
k1o=+1 of the optical beam of the first wavelength. Also, the diffraction
efficiency eff2 for the diffraction order k2o (*) of the optical beam of
the second wavelength passing through the region, for condensing light so
as to form a spot on the signal recording face of the second optical disc
via the object lens 134 is approximately zero, and the diffraction
efficiency eff3 for the diffraction order k3o (*) of the optical beam of
the third wavelength passing through the region, for condensing light so
as to form a spot on the signal recording face of the third optical disc
via the object lens 134 is approximately zero.

[0284]Next, the outer ring zone of the first embodiment will be described
in further detail with reference to FIGS. 29A through 29C. FIG. 29A is a
diagram illustrating change in the diffraction efficiency of the +1 order
diffracted light of the optical beam of the first wavelength in a case of
changing the groove depth d of the blazed form where the number of steps
S=∞, FIG. 29B is a diagram illustrating change in the diffraction
efficiency of the +2 order diffracted light of the optical beam of the
second wavelength in a case of changing the groove depth d of the blazed
form where the number of steps S=∞, and FIG. 29C is a diagram
illustrating change in the diffraction efficiency of the +2 order
diffracted light of the optical beam of the third wavelength in a case of
changing the groove depth d of the blazed form where the number of steps
S=∞. In FIGS. 29A through 29C, the horizontal axis represents the
groove depth in nm, and the vertical axis represents the diffraction
efficiency (intensity of light). As shown in FIG. 29A, at the position of
650 nm on the horizontal axis, eff1 is 0.92, eff2 is approximately zero
as shown in FIG. 29B, and eff3 is approximately zero as shown in FIG.
29C. Note that in Table 4 and the above, the diffraction orders of the
optical beams of the second and third wavelengths noted with the asterisk
"*" are +2 and +2, respectively.

[0285]Also, with the outer ring zone in the first embodiment described
above, of the design line in the relation between the above-described
(wavelength×order) and the thickness of the protective layer, the
y-intercept position and inclination with the vertical axis representing
the thickness of the protective layer as the Y axis exhibits flaring
regarding the second wavelength by change due to design of the object
lens. Accordingly, performing appropriate object lens design based on
such a design line enables the quantity of light of the optical beam of
the second wavelength to be further suppressed and excellent aperture
restriction to be performed regarding the optical beam of the second
wavelength. Specifically, as shown in FIG. 30, the outer ring zone in the
first embodiment has the design line indicated by L12 set by plotting the
points Pλ1, Pλ2, and Pλ3 at the diffraction orders
(k1o, k2o, k3o)=(+1, +2, +2). In FIG. 30 the design point Pλ1 of
the first wavelength and the design point Pλ3 of the third
wavelength are positioned on the design line L12, so the aberration of
diffraction light of the diffraction orders k1o and k3o is approximately
zero. On the other hand, the plotted point Pλ2 of the second
wavelength is significantly deviated from the aberration zero design
point, indicating the above-described flaring. Note that in FIG. 30, only
(k2o, k3o)=(2, 2) is shown plotted, but there is deviation from the
design line L12 in the same way for other orders in the second and third
wavelengths as well. Consequently, there is uncorrected aberration in the
second wavelength, and consequently, the light quantity of the optical
beam of the second wavelength which has passed through the outer ring
zone, that is not imaged at the signal recording face but input to the
second optical disc can be suppressed. As a result, a suitable aperture
restriction (NA=0.6) can be realized, regardless of the diffraction
efficiency of the second wavelength.

[0286]As described above, with the outer ring zone in the first
embodiment, the diffraction face is blazed, so according to this
configuration, even in the case of providing the diffraction units to one
face of the object lens as described later, diffraction grooves can be
formed relatively easily at the curved face of the lens face at the
perimeter of the lens which has a steep slope due to being at the outer
ring zone. Also, with the outer ring zone in the first embodiment, the
third wavelength regarding which aperture restriction the same as with
the second wavelength is desired is condensed in a state of spherical
aberration having been corrected due to selecting the +2 order, but the
diffraction efficiency is approximately zero as shown in FIG. 29C,
whereby aperture restriction functions can be manifested.

[0287]Next, description will be made regarding the second embodiment shown
in Table 5. Note that the inner ring zone in the second embodiment is of
the same configuration as that of the inner ring zone in the first
embodiment described above, as can be seen from Table 4 and Table 5, and
accordingly description thereof will be omitted.

[0288]At the middle ring zone in the second embodiment, as shown in Table
5, with a blazed form (S=∞) having a groove depth of d=2.4 (μm),
the diffraction efficiency eff1=0.96 for the diffraction order k1m=+3 of
the optical beam of the first wavelength, the diffraction efficiency
eff2=0.93 for the diffraction order k2m=+2 of the optical beam of the
second wavelength. Also, the diffraction efficiency eff3=0.48 for the
diffraction order k3m (*) of the optical beam of the third wavelength
passing through the region, for condensing light so as to form a spot on
the signal recording face of the third optical disc via the object lens
134, but as described later the spot is flared, and accordingly does not
contribute to imaging.

[0289]Next, the middle ring zone of the second embodiment will be
described in further detail with reference to FIGS. 31A through 31C. FIG.
31A is a diagram illustrating change in the diffraction efficiency of the
+3 order diffracted light of the optical beam of the first wavelength in
a case of changing the groove depth d of the blazed form where the number
of steps S=∞, FIG. 31B is a diagram illustrating change in the
diffraction efficiency of the +2 order diffracted light of the optical
beam of the second wavelength in a case of changing the groove depth d of
the blazed form where the number of steps S=∞, and FIG. 31C is a
diagram illustrating change in the diffraction efficiency of the +2 order
diffracted light of the optical beam of the third wavelength in a case of
changing the groove depth d of the blazed form where the number of steps
S=∞. In FIGS. 31A through 31C, the horizontal axis represents the
groove depth in nm, and the vertical axis represents the diffraction
efficiency (intensity of light). As shown in FIG. 31A, at the position of
2400 nm on the horizontal axis, eff1 is 0.96, eff2 is 0.93 as shown in
FIG. 31B, and eff3 is 0.48 as shown in FIG. 31C, but the spot is flared,
as described later. Note that here, the diffraction order of the optical
beam of the third wavelength noted with the asterisk "*" is +2 in Table 5
and the above description.

[0290]Also, with the middle ring zone in the second embodiment, the design
line of the object lens is changed for flaring of the third wavelength,
thereby performing excellent aperture restriction, in the same way as
with the case of the outer ring zone in the first embodiment described
above. Specifically, as shown in FIG. 33, the middle ring zone in the
second embodiment has the design line indicated by L13 set by plotting
the points Pλ1, Pλ2, and Pλ3 at the diffraction orders
(k1m, k2m, k3m)=(+3, +2, +2). In FIG. 33 the design point Pλ1 of
the first wavelength and the design point Pλ2 of the second
wavelength are positioned on the design line L13, so the aberration of
diffraction light of the diffraction orders k1m and k2m is approximately
zero. On the other hand, the plotted point Pλ3 of the third
wavelength is significantly deviated from the aberration zero design
point, indicating the above-described flaring. Note that in FIG. 33, only
k3m=+2 is shown plotted, but there is deviation from the design line L13
in the same way for other orders in the third wavelength as well.
Consequently, there is uncorrected aberration in the third wavelength,
and accordingly, the light quantity of the optical beam of the third
wavelength which has passed through the middle ring zone, that is not
imaged at the signal recording face but input to the third optical disc,
can be suppressed. As a result, even if there is a little diffraction
efficiency of the optical beam of the third wavelength, as shown in FIG.
31C, this does not contribute to the imaging of these optical beams, and
a suitable aperture restriction (NA=0.45) can be realized.

[0291]Also, the middle ring zone in the second embodiment described above
has a higher diffraction efficiency as to the first wavelength than the
middle ring zone in the first embodiment described above, and excels in
that perspective.

[0292]Also, at the outer ring zone in the second embodiment, as shown in
Table 5, with a blazed form (S=∞) having a groove depth of d=3.1
(μm), the diffraction efficiency eff1=1.0 for the diffraction order
k1o=+4 of the optical beam of the first wavelength. Also, the diffraction
efficiency eff2=0.25 for the diffraction order k2o (*) of the optical
beam of the second wavelength, for condensing light so as to form a spot
on the signal recording face of the second optical disc via the object
lens 134, but as described later the spot is flared, and accordingly does
not contribute to imaging. Further, the diffraction efficiency eff3 for
the diffraction order k3o (*) of the optical beam of the third wavelength
passing through the region, for condensing light so as to form a spot on
the signal recording face of the third optical disc via the object lens
134 is approximately zero.

[0293]Next, the outer ring zone of the second embodiment will be described
in further detail with reference to FIGS. 32A through 32C. FIG. 32A is a
diagram illustrating change in the diffraction efficiency of the +4 order
diffracted light of the optical beam of the first wavelength in a case of
changing the groove depth d of the blazed form where the number of steps
S=∞, FIG. 32B is a diagram illustrating change in the diffraction
efficiency of the +3 order diffracted light of the optical beam of the
second wavelength in a case of changing the groove depth d of the blazed
form where the number of steps S=∞, and FIG. 32C is a diagram
illustrating change in the diffraction efficiency of the +3 order
diffracted light of the optical beam of the third wavelength in a case of
changing the groove depth d of the blazed form where the number of steps
S=∞. In FIGS. 32A through 32C, the horizontal axis represents the
groove depth in nm, and the vertical axis represents the diffraction
efficiency (intensity of light). As shown in FIG. 32A, at the position of
3100 nm on the horizontal axis, eff1 is 1.0, eff2 is 0.25 as shown in
FIG. 32B, but the spot is flared as will be described later. Further,
eff3 is approximately zero as shown in FIG. 32C. Note that in Table 5 and
the above, the diffraction orders of the optical beams of the second and
third wavelengths noted with the asterisk are +3 and +3, respectively.

[0294]Also, with the outer ring zone in the second embodiment, the design
line of the object lens is changed for flaring of the second and third
wavelengths, thereby performing excellent aperture restriction, in the
same way as with the case of the outer ring zone in the first embodiment
described above. Specifically, as shown in FIG. 34, the outer ring zone
in the second embodiment has the design line indicated by L14 set by
plotting the points Pλ1, Pλ2, and Pλ3 at the
diffraction orders (k1o, k2o, k3o)=(+4, +3, +3). In FIG. 34 the design
point Pλ1 of the first wavelength and is positioned on the design
line L14, so the aberration of diffracted light of the diffraction order
k1o is approximately zero. On the other hand, the plotted points
Pλ2 and Pλ3 of the second and third wavelengths are
significantly deviated from the aberration zero design point, indicating
the above-described flaring. Note that in FIG. 34, only (k2o, k3o)=(+3,
+3) is shown plotted, but there is deviation from the design line L14 in
the same way for other orders in the second and third wavelengths as
well. Consequently, there is uncorrected aberration in the second and
third wavelengths, and accordingly, the light quantity of the optical
beams of the second and third wavelengths which has passed through the
outer ring zone, that is not imaged at the signal recording face but
input to the second and third optical discs can be suppressed. As a
result, even if there is a little diffraction efficiency of the optical
beam of the second wavelength as shown in FIG. 32, this does not
contribute to the imaging of these optical beams, and a suitable aperture
restriction (NA=0.6) can be realized. Also, an even more suitable
aperture restriction (NA=0.45) can be realized for the optical beam of
the third wavelength.

[0295]Now, while it can be said that the outer ring zone in the first
embodiment is basically easier to employ from a design perspective, there
is demand for reduced aberration change due to temperature as described
above in the diffraction unit having such an outer ring zone as described
above, and the outer ring zone in the second embodiment is advantageous
from this aspect. This will be described using the above-described effect
term ΔWn due to refractive index fluctuation of the composition
material under change in temperature, and effect term ΔWλ due
to wavelength fluctuation of the incident optical beam under change in
temperature. Generally, |ΔWn| is greater than |ΔWλ|, so
it is difficult to realize ΔW≈0 with a diffraction order
around 1 or so. Also, the effect term ΔWλ is generally
proportionate to the diffraction order, so employing as great a
diffraction order as possible can increase the ΔWλ which can
be understood as being aberration change amount occurring due to
diffraction, thereby aiming to realize ΔW≈0 of the
spherical aberration ΔW due to temperature rise. A design example
according to this perspective is the outer ring zone (k1o=+4) according
to the second embodiment described with reference to FIGS. 32A through
32C and FIG. 34, and the amount of aberration occurring at the time of
temperature change can be reduced as compared to the outer ring zone
according to the first embodiment where k1o=+1 is employed. Describing
this with a longitudinal aberration diagram in the same way as with FIG.
25 above, if we say that a longitudinal aberration diagram accompanying
the temperature change in a case of (k1i, k1m, k1o)=(+1, +1, +1) is
obtained as in FIG. 25B, in a case of selecting relatively high order
diffraction orders at the middle ring zone and the outer ring zone such
that (k1i, k1m, k1o)=(+1, +3, +4), a state such as shown in FIG. 25C is
obtained. In FIG. 25C, the dotted line Lwn is the same as in FIG. 25B,
the single-dot broken line Lwλ3 represents the effect term the
effect term ΔWλ due to wavelength fluctuation in the case of
selecting a relatively high order diffraction order for the middle ring
zone and outer ring zone, as longitudinal aberration. In FIG. 25C the
solid line Lw3 represents spherical aberration ΔW occurring due to
temperature rise, obtained by adding the effect term ΔWn and effect
term ΔWλ indicated by Lwn and Lwλ3. Thus, it can be
seen from FIG. 25C that occurrence of longitudinal aberration (Lw3) is
further suppressed as compared to the longitudinal aberration amount
shown by the solid line Lw2 in FIG. 25B.

[0296]With the diffraction unit of the second embodiment having such an
inner ring zone, middle ring zone, and outer ring zone, diffraction
efficiency as to the first wavelength in particular is excellent for all
ring zones, thereby realizing high diffraction efficiency as to the first
wavelength, for which there has been strong demand regarding
three-wavelength compatibility but which has been difficult with
compatibility lenses which have been studied with relation to the related
art.

[0297]The diffraction unit 150 and the object lens 134, having the first
through third diffraction regions 151, 152, and 153 with the
configuration such as described above, are capable of condensation of the
optical beams of the first through third wavelengths passing through the
first diffraction region 151 so as to form a suitable spot on the signal
recording face of the corresponding optical disc by being input to the
object lens 134, in a divergent angle state wherein no spherical
aberration occurs at the signal recording face of respectively
corresponding optical discs via the common object lens 34, i.e., in a
converged state wherein spherical aberration is corrected via the object
lens 134, and is capable of condensation of the optical beams of the
first and second wavelengths passing through the second diffraction
region 152 so as to form a suitable spot on the signal recording face of
the corresponding optical disc by being input to the object lens 134, in
a divergent angle state wherein no spherical aberration occurs at the
signal recording face of respectively corresponding optical discs via the
common object lens 34, i.e., in a converged state wherein spherical
aberration is corrected via the object lens 134, and also is capable of
condensation of the optical beams of the first wavelength passing through
the third diffraction region 153 so as to form a suitable spot on the
signal recording face of the corresponding optical disc by being input to
the object lens 134, in a divergent angle state wherein no spherical
aberration occurs at the signal recording face of the corresponding
optical disc via the object lens 34, i.e., in a dispersed state or
converged state wherein spherical aberration is corrected via the object
lens 134.

[0298]That is to say, the diffraction unit 150 provided on one face of the
diffraction optical element 135 disposed on the optical path between the
first through third emitting units in the optical system of the optical
pickup 103 and the signal recording face allows optical beams of
respective wavelengths passing through the respective regions (first
through third diffraction regions 151, 152, and 153) to be input to the
object lens 134 in a state wherein spherical aberration occurring at the
signal recording face to be reduced, so spherical aberration occurring at
the signal recording face when condensing optical beams of the first
through third wavelengths on the signal recording face of the respective
corresponding optical discs using the common object lens 134 in the
optical pickup 3 can be minimized, which is to say that three-wavelength
compatibility of the optical pickup 3 using three types of wavelengths
for three types of optical discs and a common object lens 134 can be
realized, wherein information signals can be recorded to and/or played
from respective optical discs.

[0299]Also, the above-described diffraction unit 150 and object lens 134
having the first through third diffraction regions 151, 152, and 153, are
configured such that the diffraction orders (k1i, k2i, k3i) of light
selected by the first diffraction region 151 serving as the inner ring
zone and condensed on the signal recording face of the corresponding
optical disk via the object lens 134 are set to (+1, +1, +1), light can
be condensed on the signal recording face of each optical disc in a state
of the three wavelengths having spherical aberration reduced and with a
high diffraction efficiency for each, i.e., with sufficient light
quantity, and also, spherical aberration occurring due to change in
temperature reduced, and further, the groove depth of the diffraction
structure to be formed can be prevented from becoming too deep so
manufacturing is easily, and the problem of deterioration in precision
and so forth is prevented, thereby obtaining a configuration which is
advantageous from the perspective of manufacturing.

[0300]Further, the diffraction unit 150 and object lens 134 are configured
such that the diffraction orders (k1m, k2m) of light selected by the
second diffraction region 152 serving as the middle ring zone and
condensed on the signal recording face of the corresponding optical disk
via the object lens 134 are set to (+1, +1) or (+3, +2), light can be
condensed on the signal recording face of each optical disc in a state of
the first and second wavelengths having spherical aberration reduced and
with sufficient light quantity, and also, spherical aberration occurring
due to change in temperature reduced, thereby obtaining a configuration
which is advantageous from the perspective of manufacturing, and further,
advantages of the above-described flaring can be obtained as well.

[0301]Moreover, the diffraction unit 150 and object lens 134 are
configured such that the diffraction order k1o of light selected by the
third diffraction region 153 serving as the outer ring zone and condensed
on the signal recording face of the corresponding optical disk via the
object lens 134 is set to +1, +2, +3, +4, +5, so light can be condensed
on the signal recording face of each optical disc in a state of the first
wavelength having spherical aberration reduced and with sufficient light
quantity, and also, spherical aberration occurring due to change in
temperature reduced, thereby obtaining a configuration which is
advantageous from the perspective of manufacturing, and further,
advantages of the above-described flaring can be obtained as well.

[0302]Also, the diffraction unit 150 having the first through third
diffraction regions 151, 152, and 153 is capable of suitably solving the
problems of diffraction efficiency and spherical aberration at the time
of temperature change, of which solving has been difficult with
three-wavelength compatible lenses studied with relation to the related
art. That is, with the three-wavelength compatible lenses studied with
relation to the related art, raising the design efficiency of the first
wavelength which is the shortest wavelength has been difficult, and
further the curvature at the lens perimeter is great due to being a
three-wavelength compatible lens so there has been the problem such as
necessary diffraction efficiency not being able to be obtained when
diffraction efficiency drops due to the precision in form of the
diffraction structure formed at the perimeter portion being low, and the
problem that even if aberration can be suppressed when diffraction orders
of opposite signs are selected for the first through third wavelengths,
aberration increases for wavelengths regarding which diffraction orders
of opposite signs are selected, due to inversion in behavior at the time
of temperature changing between diffraction orders of opposite signs
being selected for the first through third wavelengths, and that
generally with such diffraction units, the amount of spherical aberration
occurring due to the refraction index at the time of temperature rising
is cancelled out by the amount of spherical aberration occurring due to
wavelength fluctuation at the time of temperature rising, and that the
sign of effect of the amount of spherical aberration occurring due to
wavelength fluctuation at the time of temperature rising is determined by
the diffraction direction; however, with the above-described diffraction
unit 150 having the first through third diffraction regions 151, 152, and
153, the design efficiency as to the first wavelength can be raised to
almost 100%, and also occurrence of spherical aberration at the time of
temperature change can be suppressed.

[0303]Further, by forming the first diffraction region 151 of the
diffraction unit 150 with a blazed form having a shallow groove depth to
realize three-wavelength compatibility, the manufacturing processing
becomes easy, enabling simplification of manufacturing and reduction in
costs, and particularly, the case of integrating the diffraction unit
with the object lens as described later, a configuration advantageous
from the perspective of manufacturing can be obtained. Also, by forming
the second and third diffraction regions 152 and 153 of the diffraction
unit 150 with a blazed form having a shallow groove depth, the
manufacturing processing becomes easy, enabling simplification of
manufacturing and reduction in costs, and particularly, the case of
integrating the diffraction unit with the object lens as described later,
a configuration advantageous from the perspective of manufacturing can be
obtained.

[0304]Also, the diffraction unit 150 having the first through third
diffraction regions 151, 152, and 153 is configured such that an order
other than the diffraction order, whereby the optical beam of the third
wavelength passing through the second and third diffraction regions 152
and 153 is suitably condensed on the signal recording face of the
corresponding type of optical disc via the object lens 134, is dominant,
so that only the portion of the optical beam which has passed through the
first diffraction region 151 is condensed on the signal recording face of
the optical disc via the object lens 134, and the first diffraction
region 151 is formed to a size such that the optical beam of the third
wavelength passing through this region is shaped to have a size of a
predetermined numerical aperture, whereby aperture restriction can be
performed regarding the optical beam of the third wavelength so as to
have a numerical aperture of around 0.45, for example. Note that by
forming the diffraction unit 150 and object lens 134 such that flaring is
implemented regarding the third wavelength as described at one or both of
the second and third diffraction regions 152 and 153, whereby the light
quantity of the optical beam of third wavelength condensed on the signal
recording face of the third optical disc is further suppressed, thereby
enabling manifesting of further aperture restriction functions.

[0305]Also, the diffraction unit 150 is configured such that an order
other than the diffraction order, whereby the optical beam of the second
wavelength passing through the third diffraction region 153 is suitably
condensed on the signal recording face of the corresponding type of
optical disc via the object lens 134, is dominant, so that only the
portion of the optical beam which has passed through the first and second
diffraction regions 151 and 152 is condensed on the signal recording face
of the optical disc via the object lens 134, and the first and second
diffraction regions 151 and 152 are formed to a size such that the
optical beam of the second wavelength passing through this region is
shaped to have a size of a predetermined numerical aperture, whereby
aperture restriction can be performed regarding the optical beam of the
second wavelength so as to have a numerical aperture of around 0.60, for
example. Note that by forming the diffraction region 150 and object lens
134 such that flaring is implemented regarding the second wavelength as
described at the third diffraction region 153, whereby the light quantity
of the optical beam of the second wavelength condensed on the signal
recording face of the second optical disc is further suppressed, thereby
enabling manifesting of further aperture restriction functions.

[0306]Also, the diffraction unit 150 performs places the optical beam of
the first wavelength passing outside of the third diffraction region 153
in a state so as to not be suitably condensed on the signal recording
face of the corresponding type of optical disc via the object lens 134,
or shields the optical beam of the first wavelength passing outside of
the third diffraction region 153, whereby, with regard to the optical
beam of the first wavelength, only the optical beam portion which has
passed through the first through third diffraction regions 151, 152, and
153 is condensed on the signal recording face of the optical disc via the
object lens 134, and also, the first through third diffraction regions
151, 152, and 153 are formed to a size which is the numerical aperture of
the first wavelength optical beam passing through this region, whereby
aperture restriction can be performed regarding the optical beam of the
first wavelength such that NA= around 0.85, for example.

[0307]Thus, the diffraction unit 150 provided on one face of the
diffraction optical element 135 disposed on the optical path as described
above not only realizes three-wavelength compatibility, but also enables
optical beams of each wavelength to be input to the common object lens
134 in a state wherein aperture restriction is performed appropriately
with a numerical aperture corresponding to each of the three types of
optical discs and optical beams of the first through third wavelengths.
Thus, the diffraction unit 150 not only has functions of aberration
correction corresponding to the three wavelengths, but also has functions
as an aperture restricting unit.

[0308]It should be noted that a diffraction unit can be configured by
suitably combining the diffraction regions in the above-described
embodiments. That is to say, the diffraction order of each wavelength
passing through each diffraction region can be selected as appropriate.
In the event of changing the diffraction order of each wavelength passing
through each diffraction region, an object lens 134 corresponding to each
diffraction order of each wavelength passing through each region can be
used.

[0309]Also, while description has been made above with the diffraction
unit 150 configured of the three diffraction regions 151, 152, and 153
formed on the incident side face of the diffraction optical element 135
provided separately from the object lens 134, as shown in FIG. 35A, the
present invention is not restricted to this arrangement, and may be
provided to the output side face of the diffraction optical element 135.
Further, the diffraction unit 150 having the first through third
diffraction regions 151, 152, and 153, can be integrally configured on
the input or output side face of the object lens 134, or, as shown in
FIG. 35B for example, an object lens 134B having the diffraction unit 150
on the incident side face thereof may be configured. In the event of
providing the diffraction unit 150 on the incident side face of the
object lens 134B for example, the planar shape of the above-described
diffraction structure is combined with a reference face at the incident
side face required for the lens to be able to function as an object lens.
While the above-described diffraction optical element 135 and the object
lens 134 are two separate elements serving as a condensing optical
device, the object lens 134B thus configured functions as a condensing
optical device which can perform suitable light condensing such that
spherical aberration does not occur at the signal recording face of
optical discs corresponding to each of the three optical beams of
different wavelengths, with a single element. Providing the diffraction
unit 150 integrally with the object lens 134B enables further reduction
in optical parts and also reduction in configuration size. The object
lens 134B having a diffraction unit having functions the same as the
diffraction unit 150 provided integrally at the input side or output side
face realizes three-wavelength compatibility of the optical pickup by
reducing aberration and so forth when used in an optical pickup, and also
reduces the number of parts so as to enable simplification and reduction
in size of the configuration, thereby realizing high production and
reduced costs. Note that the above-described diffraction unit 150
sufficiently manifests the advantages thereof with the diffraction
structure for aberration correction to realize three-wavelength
compatibility being provided on a single face that has been difficult
with the related art, which enables such a diffraction element to be
integrally formed with the object lens 134 serving as such a refractive
element, further enabling directly forming a diffraction face on a
plastic lens, and forming the object lens 134B with which the diffraction
unit 150 has been integrated of a plastic material further realizing
improved production and lower costs.

[0310]The collimator lens 142 provided between the diffraction optical
element 135 and the third beam splitter 138 converts the divergent angle
of each of the first through third wavelength optical beams of which the
optical paths have been synthesized at the second beam splitter 137 and
passed through the third beam splitter 138, and outputs to the
quarter-wave plate 143 and diffraction optical element 135 side, in a
generally parallel light state, for example. The arrangement wherein the
collimator lens 142 inputs the optical beams of the first and second
wavelengths into the above-described diffraction optical element 135 with
the divergent angle thereof in the state of generally parallel light, and
also inputs the optical beam of the third wavelength into the diffraction
optical element 135 with divergent angle in a state which is slightly
diffused as to parallel light (hereinafter also referred to as "finite
system state") enables further reduction of spherical aberration,
slightly occurring at the time of condensing the third wavelength optical
beam on the signal recording face of the third optical disc via the
diffraction optical element 135 and the object lens 134, described with
reference to FIG. 24, to realize three-wavelength compatibility with less
aberration occurring. While an arrangement has been described here
wherein the optical beam of the third wavelength is input to the
diffraction optical element 135 in a state of a predetermined divergent
angle, due to the positional relation between the third light source 133
having the third emitting unit for emitting the third wavelength optical
beam and the collimator lens 142, in the event of positioning multiple
emitting units at a common light source for example, this may be realized
by providing an element which converts only the divergent angle of the
optical beam of the third wavelength, or by inputting into the
diffraction optical element 135 in a predetermined divergent angle state
by providing a mechanism to drive the collimator lens 142, or the like.
Also, the optical beam of the second wavelength, or the optical beams of
the second and third wavelengths, may be input to the diffraction optical
element 135 in the finite system state, thereby further reducing
aberration. Also, optical beams of the second and third wavelengths may
be input in the finite system state and in a diffused state, thereby
realizing adjustment of return power and even more excellent optical
system compatibility may be achieved by setting the focus capture range
and so forth to a desired state matching the format by adjusting the
return power. Note that in this case, the object lens 134 is formed with
the design line situated downwards by a predetermined distance with
regard to the plotted points Pλ2 and Pλ3 with regard to the
second and third wavelengths in the relation between the
wavelength×diffraction order and protective layer thickness
described above.

[0311]The multi-lens 146 is, for example, a wavelength-selective
multi-lens, whereby the returning first through third wavelength optical
beams separated from the outgoing path optical beams by being reflected
at the third beam splitter 138, after having been reflected off of the
signal recording face of the respective optical disc, and passed through
the object lens 134, diffraction optical element 135, redirecting mirror
144, quarter-wave plate 143, and collimator lens 142, is appropriately
condensed on the photoreception face of the photodetector or the like of
the photosensor 145. At this time, the multi-lens 146 provides the return
optical beam with astigmatism for detection of focus error signals or the
like.

[0312]The photosensor 145 receives the return optical beam condensed at
the multi-lens 146, and detects, along with information signals, various
types of detection signals such as focus error signals, tracking error
signals, and so forth.

[0313]With the optical pickup 103 configured as described above, the
object lens 134 is driven so as to be displaced based on the focus error
signals and tracking error signals obtained by the photosensor 145,
whereby the object lens 134 is moved to a focal position as to the signal
recording face of the optical disc 2, the optical beam is focused onto
the signal recording face of the optical disc 2, and information is
recorded to or played from the optical disc 2.

[0314]The optical pickup 103 is provided on one face of the diffraction
optical element 135, can provide optical beams of each wavelength with a
diffraction efficiency and diffraction angle suitable for each region due
to the diffraction unit 150 having the first through third diffraction
regions 151, 152, and 153, can sufficiently reduce spherical aberration
at the signal recording face of the three types of first through third
optical discs 11, 12, and 13, of which the format for the thickness of
the protective layer or the like differs, and enables reading and writing
of signals to and from the multiple types of optical discs 11, 12, and
13, using optical beams of three different wavelengths.

[0315]Also, the diffraction optical element 135 having the diffraction
unit 150, and object lens 134, in the above optical pickup 103, can
function as a condensing optical device for condensing incident optical
beams at a predetermined position. In the event of using an optical
pickup which performs recording and/or playing of information signals by
irradiating optical beams onto three different types of optical discs,
the diffraction unit 150 provided on one face of the diffraction optical
element 135 enables the condensing optical device to appropriately
condense corresponding optical beams onto the signal recording face of
the three types of optical discs in a state with spherical aberration
sufficiently reduced, meaning that three-wavelength compatibility of the
optical pickup using the object lens 134 common to the three wavelengths
can be realized.

[0316]Also, while description has been made above regarding a
configuration wherein the diffraction optical element 135 to which the
diffraction unit 150 is provided, and the object lens 134, are provided
to an actuator such as an object lens driving mechanism or the like for
driving the object lens 134 is as to be integral, this may be configured
as a condensing optical unit wherein the diffraction optical element 135
and the object lens 134 are formed as an integrated unit, in order to
improve precision of assembly to the lens holder of the actuator, and
facilitate assembly work. For example, a condensing optical unit can be
configured by use spacers or the like to fix the diffraction optical
element 135 and object lens 134 to the holder while setting the
positioning, spacing, and optical axis, so as to be integrally formed.
Due to being integrally assembled to the object lens driving mechanism as
described above, the diffraction optical element 135 and object lens 134
can appropriately condense the first through third wavelength optical
beams on the signal recording face of the respective optical discs in a
state with spherical aberration reduced, even at the time of field shift
such as displacement in the tracking direction.

[0317]Next, the optical paths of the optical beams emitted from the first
through third light sources 131, 132, and 133 of the optical pickup 103
configured as described above, will be described with reference to FIG.
2. First, the optical path at the time of emitting the optical beam of
the first wavelength as to the first optical disc 11 and performing
reading or writing of information will be described.

[0318]The disc type determination unit 22 which has determined that the
type of the optical disc 2 is the first optical disc 11 causes the
optical beam of the first wavelength to be emitted from the first
emitting unit of the first light source 131.

[0319]The optical beam of the first wavelength emitted from the first
emitting unit is split into three beams by the first grating 139, for
detection of tracking error signals and so forth, and is input to the
second beam splitter 137. The optical beam of the first wavelength which
has been input to the second beam splitter 137 is reflected at a mirror
face 137a thereof, and is output to the third beam splitter 138 side.

[0320]The optical beam of the first wavelength which is input to the third
beam splitter 138 is transmitted through a mirror face 138a thereof,
output to the collimator lens 142 side, where the divergent angle is
converted so as to be generally parallel light by the collimator lens
142, provided with a predetermined phase difference at the quarter-wave
plate 143, reflected off of the redirecting mirror 144, and output to the
diffraction optical element 135 side.

[0321]The optical beam of the first wavelength which is input to the
diffraction optical element 135 is output with the optical beam which has
passed through each region thereof having a predetermined diffraction
order (k1i, k1m, k1o) dominant therein as described above, due to the
first through third diffraction regions 151, 152, and 153 of the
diffraction unit 150 provided on the incident side face thereof, and
input to the object lens 134. The optical beam of the first wavelength
output from the diffraction optical element 135 is not only in a state of
a predetermined divergent angle, but also is in a state of aperture
restriction.

[0322]The optical beam of the first wavelength input to the object lens
134 has been input in a converged state of the divergent angle whereby
spherical aberration of the optical beam having passed through the
regions 151, 152, and 153 can be reduced, and accordingly is
appropriately condensed by the object lens 134 on the signal recording
face of the first optical disc 11.

[0323]The optical beam condensed at the first optical disc 11 is reflected
at the signal recording face, passes through the object lens 134,
diffraction optical element 135, redirecting mirror 144, quarter-wave
plate 143, and collimator lens 142, is reflected off of the mirror face
138a of the third beam splitter 138, and is output to the photosensor 145
side.

[0324]The optical beam split from the optical path of the outgoing optical
beam reflected off of the third beam splitter 138 is condensed on the
photoreception face of the photosensor by the multi-lens 146, and
detected.

[0325]Next, description will be made regarding the optical path at the
time of emitting an optical beam of the second wavelength to the second
optical disc 12 and reading or writing information. The disc type
determination unit 22 which has determined that the type of the optical
disc 2 is the second optical disc 12 causes the optical beam of the
second wavelength to be emitted from the second emitting unit of the
second light source 132.

[0326]The optical beam of the second wavelength emitted form the second
emitting unit is split into three beams by the second grating 140, for
detection of tracking error signals and so forth, and is input to the
first beam splitter 136. The optical beam of the second wavelength which
has been input to the first beam splitter 136 is transmitted through a
mirror face 136a thereof, also transmitted through the mirror face 137a
of the second beam splitter 137, and is output to the third beam splitter
138 side.

[0327]The optical beam of the second wavelength which is input to the
third beam splitter 138 is transmitted through the mirror face 138a
thereof, output to the collimator lens 142 side, where the divergent
angle is converted so as to be generally parallel light or diffused
light, by the collimator lens 142, provided with a predetermined phase
difference at the quarter-wave plate 143, reflected off of the
redirecting mirror 144, and output to the diffraction optical element 135
side.

[0328]The optical beam of the second wavelength which is input to the
diffraction optical element 135 is output with the optical beam which has
passed through each region thereof having a predetermined diffraction
order dominant therein as described above, due to the first through third
diffraction regions 151, 152, and 153 of the diffraction unit 150
provided on the incident side face thereof, and input to the object lens
134. The optical beam of the second wavelength output from the
diffraction optical element 135 is not only in a state of a predetermined
divergent angle, but also is in a state of aperture restriction due to
being input to the object lens 134.

[0329]The optical beam of the second wavelength input to the object lens
134 has been input in a divergent angle state whereby spherical
aberration of the optical beam having passed through the first and second
diffraction regions 151 and 152 can be reduced, and accordingly is
appropriately condensed by the object lens 134 on the signal recording
face of the second optical disc 12.

[0330]The return side optical path of the optical beam reflected off of
the signal recording face of the second optical disc 12 is the same as
with the case of the above-described optical beam of the first
wavelength, and accordingly description thereof will be omitted.

[0331]Next, description will be made regarding the optical path at the
time of emitting an optical beam of the third wavelength to the third
optical disc 13 and reading or writing information. The disc type
determination unit 22 which has determined that the type of the optical
disc 2 is the third optical disc 13 causes the optical beam of the third
wavelength to be emitted from the third emitting unit of the third light
source 133.

[0332]The optical beam of the third wavelength emitted from the third
emitting unit is split into three beams by the third grating 141, for
detection of tracking error signals and so forth, and is input to the
first beam splitter 136. The optical beam of the third wavelength which
has been input to the first beam splitter 136 is reflected off of the
mirror face 136a thereof, transmitted through the mirror face 137a of the
second beam splitter 137, and is output to the third beam splitter 138
side.

[0333]The optical beam of the third wavelength which is input to the third
beam splitter 138 is transmitted through the mirror face 138a thereof,
output to the collimator lens 142 side, where the divergent angle is
converted by the collimator lens 142 so as to be diffused as to generally
parallel light, provided with a predetermined phase difference at the
quarter-wave plate 143, reflected off of the redirecting mirror 144, and
output to the diffraction optical element 135 side.

[0334]The optical beam of the third wavelength which is input to the
diffraction optical element 135 is output with the optical beam which has
passed through each region thereof having a predetermined diffraction
order dominant therein as described above, due to the first through third
diffraction regions 151, 152, and 153 of the diffraction unit 150
provided on the incident side face thereof, and input to the object lens
134. The optical beam of the third wavelength output from the diffraction
optical element 135 is not only in a state of a predetermined divergent
angle, but also is in a state of aperture restriction due to having been
input to the object lens 134.

[0335]The optical beam of the third wavelength input to the object lens
134 has been input in a divergent angle state whereby spherical
aberration of the optical beam having passed through the first
diffraction region 151 can be reduced, and accordingly is appropriately
condensed by the object lens 134 on the signal recording face of the
third optical disc 13.

[0336]The return side optical path of the optical beam reflected off of
the signal recording face of the third optical disc 13 is the same as
with the case of the above-described optical beam of the first
wavelength, and accordingly description thereof will be omitted.

[0337]Note that while a configuration has been described here wherein the
optical beam of the third wavelength has the position of the third
emitting unit adjusted such that the optical beam of which the divergent
angle is converted by the collimator lens 142 and input to the
diffraction optical element 135 is in a diffused state as to a state of
generally parallel light, a configuration may be made wherein the optical
beam is input to the diffraction optical element 135 by providing an
element which has wavelength selectivity and converts the divergent
angle, or by providing a mechanism which drives the collimator lens 142
in the optical axis direction.

[0338]Also, while description has been made regarding a configuration
wherein the optical beam of the first wavelength is input to the
diffraction optical element 135 in a state of generally parallel light,
the optical beam of the second wavelength is input to the diffraction
optical element 135 in a state of generally parallel light or diffused
light, and the optical beam of the third wavelength is input to the
diffraction optical element 135 in a diffused state, the present
invention is not restricted to this arrangement, and configurations may
be made wherein, for example, the first through third wavelength optical
beams are selectively input to the diffraction optical element 135 in a
state of diffused light, parallel light, or converged light, taking into
consideration the diffraction order selected according to the design line
of the object lens 134 and diffraction unit 150.

[0339]The optical pickup 103 to which the present invention has been
applied has first through third emitting units for emitting optical beams
of first through third wavelengths, an object lens 134 for condensing the
optical beams of first through third wavelengths emitted from the first
through third emitting units into a signal recording face of an optical
disc, and a diffraction unit 150 provided on one face of an optical
element disposed on the outgoing optical path of the optical beams of
first through third wavelengths, wherein the diffraction unit 150 has
first through third diffraction regions 151, 152, and 153, with the first
through third diffraction regions 151, 152, and 153 being different
diffraction structures circular in shape and having a predetermined
depth, and the first through third diffraction structures whereby optical
beams of each wavelength are diffracted such that diffracted light of a
predetermined diffraction order is dominant as described above, and
according to this configuration, optical beams corresponding to each of
three types of optical discs having difference usage wavelengths can be
appropriately condensed on the signal recording face using the common
object lens 134, thereby realizing excellent recording and/or playing of
information signals to/from the respective optical discs by realizing
three-wavelength compatibility with the common object lens 134, without
necessitating a complex structure.

[0340]That is to say, the optical pickup 103 to which the present
invention has been applied obtains optimal diffraction efficiencies and
diffraction angels for the first through third wavelength optical beams
due to the diffraction unit 150 provided on one face within the optical
path thereof, whereby signals can be read from and written to the
multiple types of optical discs 11, 12, and 13, using the optical beams
of different wavelengths emitted from the multiple emitting units
provided to each of the light sources 131, 132, and 133, and also optical
parts such as the object lens 134 and so forth can be shared, thereby
reducing the number of parts, simplifying and reducing the size of the
configuration, and realizing high production and lower costs.

[0341]The optical pickup 103 to which the present invention has been
applied is configured with the diffraction unit 150 and object lens 134
has the predetermined diffraction orders (k1i, k2i, k3i) selected by the
first diffraction region 151 set to (+1, +1, +1), whereby light can be
condensed on the signal recording face of each optical disc with
sufficiently high light use efficiency while reducing spherical
aberration to the three wavelengths, and also excellent spherical
aberration properties at the time of temperature change can be obtained,
thereby realizing excellent compatibility and realizing excellent
recording and/or playing to/from each optical disc.

[0342]The optical pickup 103 to which the present invention has been
applied is configured with the diffraction unit 150 and object lens 134
has the predetermined diffraction orders (k1m, k2m) selected by the
second and/or third diffraction regions 152 and 153 set to (+1, +1) or
(+3, +2) and k1o set to +1, +2, +3, +4, and +5, whereby light can be
condensed on the signal recording face of each optical disc with
sufficiently high light use efficiency while reducing spherical
aberration to the corresponding wavelengths, and with particularly high
light use efficiency regarding the optical beam of the first wavelength,
and also even more excellent spherical aberration properties at the time
of temperature change can be obtained, thereby realizing even more
excellent compatibility and realizing excellent recording and/or playing
to/from each optical disc.

[0343]Also, the optical pickup 103 to which the present invention has been
applied can share the object lens 134 between the three wavelengths,
thereby preventing trouble of reduction of sensitivity of the actuator
and so forth due to increase weight of moving parts. Also, the optical
pickup 103 to which the present invention has been applied can
sufficiently reduce spherical aberration which is problematic in the case
of sharing the object lens 134 between the three wavelengths, due to the
diffraction unit 150 provided on one face of the optical element, so
problems such as positioning of diffraction units one to another in the
event that multiple diffraction units are provided on multiple faces to
reduce spherical aberration as with the related art, and deterioration of
diffraction efficiency due to providing of the multiple diffraction
units, can be prevented, which realizes simplification of the assembly
process and improved usage efficiency of light. Also, with the optical
pickup 103 to which the present invention has been applied, a
configuration wherein the diffraction unit 150 is provided on one face of
the optical element as described above enables a configuration having an
object lens 134B including the diffraction unit 150 instead of the object
lens 134 and the diffraction optical element 135, and by integrally
forming the diffraction unit 150 with the object lens, realizes further
simplification of the structure, reduction in weight of moving parts of
the actuator, simplification of the assembly process, and improved usage
efficiency of light.

[0344]Further, the optical pickup 103 to which the present invention has
been applied not only realizes three-wavelength compatibility with the
diffraction unit 150 provided on the one face of the diffraction optical
element described above, but also can perform aperture restriction with a
numerical aperture corresponding to each of the three types of optical
discs and optical beams of three types, thereby doing away with the need
for aperture restriction filters or the like which have been necessary
with the related art, and also adjustment in the positioning thereof,
which enables further simplification of configuration, reduction in size,
and reduction in costs. Also, the optical pickup 103 has a configuration
wherein the above-described flaring is enabled at one or both of the
second and third diffraction regions 152 and 153 at the diffraction unit
150 and object lens 134, thereby manifesting even more excellent aperture
restriction functions.

[0345]Also, while the above optical pickup 103 has been described having
the first emitting unit provided at the first light source 131, the
second emitting unit provided at the second light source 132, and the
third emitting unit provided at the third light source 133, the present
invention is not restricted to this arrangement, and an arrangement may
be made wherein a light source having two of the first through third
emitting units, and another light source having the remaining one
emitting unit, are provided at different positions.

[0346]Next, description will be made regarding an optical pickup 160 shown
in FIG. 36 including a light source having a first emitting unit, and a
light source having second and third emitting units. Note that portions
in the following description which are the same as with the optical
pickup 103 will be denoted with the same reference numerals, and
description thereof will be omitted.

[0347]As shown in FIG. 36, the optical pickup 160 to which the present
invention has been applied includes a first light source 161 having a
first emitting unit for emitting an optical beam of a first wavelength, a
second light source 162 having a second emitting unit for emitting an
optical beam of a second wavelength and a third emitting unit for
emitting an optical beam of a third wavelength, an object lens 134 for
condensing optical beams emitted from the first through third emitting
units onto the signal recording face of an optical disc 2, and a
diffraction optical element 135 provided on the optical path between the
first through third emitting units and the object lens 134. This
diffraction optical element 135 is provided with the diffraction unit
150, as described above. Also, with the optical pickup 160 described here
as well, a configuration may be made wherein the diffraction unit 150 is
integrally provided on one face of the optical lens, either the input
side or output side, such as with the above-described object lens 134B
for example, instead of the object lens 134 and the diffraction optical
element 135.

[0348]Also, the optical pickup 160 includes a beam splitter 163 serving as
an optical path synthesizing unit for synthesizing the optical paths of
the optical beam of the first wavelength that has been emitted from the
first emitting unit of the first light source 161 and the optical beams
of the second and third wavelengths that have been emitted from the
second and third emitting unit of the second light source 162, and a beam
splitter 164 serving the same function as the above third beam splitter
138.

[0349]Further, the optical pickup 160 has a first grating 139, and a
grating 165 with wavelength dependency, provided between the second light
source unit 162 and the beam splitter 163, for diffracting the optical
beams of the second and third wavelengths that have been emitted from the
second and third emitting units into three beams, for detection of
tracking error signals and so forth.

[0350]Also, the optical pickup 160 has a collimator lens 142, quarter-wave
plate 143, redirecting mirror 144, photosensor 145, and multi-lens 146,
and also a collimator lens driving unit 166 for driving the collimator
lens 142 in the direction of the optical axis. The collimator lens
driving unit 166 can adjust the divergent angle of optical beams passing
through the collimator lens 142 as described above by driving the
collimator lens 142 in the direction of the optical axis, whereby not
only can spherical aberration be reduced by inputting the optical beams
to the diffraction optical element 135 and object lens 134 in a desired
state, but in the event that the mounted optical disc is a so-called
multi-layer optical disc having multiple signal recording faces,
recording and/or playing to/from each of the signal recording faces is
enabled.

[0351]With the optical pickup 160 configured as described above, the
functions of each of the optical parts is the same as with the optical
pickup 103 except for those mentioned above, and the optical paths of the
optical beams of the first through third wavelengths emitted from the
first through third emitting units are the same as with the optical
pickup 103 except for the above-mentioned, i.e., following synthesizing
of the optical paths of the optical beams of each wavelength by the beam
splitter 164, so detailed description thereof will be omitted.

[0352]The optical pickup 160 to which the present invention has been
applied has first through third emitting units for emitting optical beams
of first through third wavelengths, an object lens 134 for condensing the
optical beams of first through third wavelengths emitted from the first
through third emitting units into a signal recording face of an optical
disc, and a diffraction unit 150 provided on one face of an optical
element disposed on the outgoing optical path of the optical beams of
first through third wavelengths, wherein the diffraction unit 150 has
first through third diffraction regions 151, 152, and 153, with the first
through third diffraction regions 151, 152, and 153 being different
diffraction structures circular in shape and having a predetermined
depth, and the first through third diffraction structures whereby optical
beams of each wavelength are diffracted such that diffracted light of a
predetermined diffraction order is dominant as described above, and
according to this configuration, optical beams corresponding to each of
three types of optical discs having different usage wavelengths can be
appropriately condensed on the signal recording face using the single
shared object lens 134, thereby realizing excellent recording and/or
playing of information signals to/from the respective optical discs by
realizing three-wavelength compatibility with the common object lens 134,
without necessitating a complex structure. The optical pickup 160 also
has the other advantages of the above-described optical pickup 103, as
well.

[0353]Further, the optical pickup 160 is configured such that the second
and third emitting units are positioned at a common light source 162,
thereby realizing further simplification of configuration and reduction
in size. Note that in the same way, with the optical pickup to which the
present invention has been applied, the first through third emitting
units may be positioned at a light source at generally the same position,
thereby realizing further simplification of configuration and reduction
in size with such a configuration.

[0354]The optical disc device 1 to which the present invention has been
applied has a driving unit for holding and rotationally driving an
optical disc arbitrarily selected from the first through third optical
discs, and an optical pickup for performing recording and/or playing of
information signals from/to the optical disc being rotationally driven by
the driving unit by selectively irradiating one of multiple optical beams
of different wavelengths corresponding to the optical disc, and by using
the above-described optical pickups 103 or 160 as the optical pickup,
optical beams corresponding to each of three types of optical discs
having different usage wavelengths can be appropriately condensed on the
signal recording face due to the diffraction unit provided on one face of
the optical element on the optical path of the optical beams of the first
through third wavelengths, using a single common object lens 134, thereby
realizing excellent recording and/or playing of information signals
to/from the respective optical discs by realizing three-wavelength
compatibility with the common object lens 134, while enabling
simplification of the configuration and reduction in size, without
necessitating a complex structure.

<4>Third Embodiment of Optical Pickup (FIGS. 37 through 59)

[0355]Next, an optical pickup 203 to which the present invention is
applied will be described in detail as a third embodiment of the optical
pickup used in the above-described optical disc device 1, with reference
to FIGS. 37 through 59. As described above, the optical pickup 203 is an
optical pickup which selectively irradiates multiple optical beams onto
optical discs selected from first through third optical discs 11, 12, and
13, of which the format such as the thickness of the protective layer
differs, thereby performing recording and/or playing of information
signals.

[0356]Note that the optical pickup 203 serving as the third embodiment
described here is for solving the same problems as those of the
above-mentioned optical pickups 3 and 103, and additionally, solving the
following problems, and includes an arrangement for obtaining more
advantageous effects. Firstly, with the optical pickup 203, demands for
realizing enhancement of light use efficiency can be handled, and a
problem for reducing focal distance as to the first wavelength while
holding a suitable operating distance of the third wavelength can be
solved, and with regard to these points, the optical pickup 203 excels
the above-mentioned optical pickup 3. Secondly, with the optical pickup
203, demands for reducing unwanted light incidence can be handled, and a
problem for optimizing operating distance and focal distance can be
solved by changing the order of diffraction selected for the first and
third wavelengths, and with regard to these points, the optical pickup
203 excels the above-mentioned optical pickup 103.

[0357]As shown in FIG. 37, the optical pickup 203 to which the present
invention has been applied includes a first light source 231 having a
first emitting unit for emitting an optical beam of a first wavelength, a
second light source 232 having a second emitting unit for emitting an
optical beam of a second wavelength which is longer than the first
wavelength, a third light source 233 having a third emitting unit for
emitting an optical beam of a third wavelength which is longer than the
second wavelength, an object lens 234 which serves as a condensing
optical device for condensing optical beams emitted from the first
through third emitting units onto the signal recording face of an optical
disc 2.

[0358]Also, the optical pickup 203 includes a first beam splitter 236
provided between the second and third emitting units and the object lens
234, serving as an optical path synthesizing unit for synthesizing the
optical paths of the optical beam of the second wavelength that has been
emitted from the second emitting unit and the optical beam of the third
wavelength that has been emitted from the third emitting unit, a second
beam splitter 237 provided between the first beam splitter 236 and the
object lens 234, serving as an optical path synthesizing unit for
synthesizing the optical paths of the optical beams of the second and
third wavelengths of which the optical paths have been synthesized by the
first beam splitter 236, and the optical path of the optical beam of the
first wavelength that has been emitted from the first emitting unit, and
a third beam splitter 238 provided between the second beam splitter 237
and the object lens 234, serving as an optical path splitting unit for
splitting the outgoing optical path of the optical beam of the first
through third wavelengths of which the optical paths have been
synthesized at the second beam splitter 237 from the returning optical
path of the optical beam of the first through third wavelengths reflected
at the optical disc (hereinafter also referred to as "return path").

[0359]Further, the optical pickup 203 has a first grating 239 provided
between the first emitting unit of the first light source unit 231 and
the second beam splitter 237, for diffracting the optical beam of the
first wavelength that has been emitted from the first emitting unit into
three beams, for detection of tracking error signals and so forth, a
second grating 240 provided between the second emitting unit of the
second light source unit 232 and the first beam splitter 236, for
diffracting the optical beam of the second wavelength that has been
emitted from the second emitting unit into three beams, for detection of
tracking error signals and so forth, and a third grating 241 provided
between the third emitting unit of the third light source unit 233 and
the first beam splitter 236, for diffracting the optical beam of the
third wavelength that has been emitted from the third emitting unit into
three beams, for detection of tracking error signals and so forth.

[0360]Also, the optical pickup 203 has a collimator lens 242 provided
between the third beam splitter 238 and the object lens 234, serving as a
divergent angle conversion unit for converting the divergent angle of the
optical beam of the first through third wavelength of which the optical
paths have been synthesized at the third beam splitter 238 so as to be
adjusted into a state of generally parallel light or a state diffused or
converged as to generally parallel light, and outputting, a quarter-wave
plate 243 provided between the collimator lens 242 and the object lens
234, so as to provide quarter-wave phase difference to the optical beam
of the first through third wavelength of which the divergent angle has
been adjusted, and a redirecting mirror 244 provided between the object
lens 234 and the quarter-wave plate 243, for redirecting the optical beam
which has passed through the above-described optical parts within a plane
generally orthogonal to the optical axis of the object lens 234, so as to
emit the optical beam in the optical axis direction of the object lens
234.

[0361]Further, the optical pickup 203 includes a photosensor 245 for
receiving and detecting the optical beams of the first through third
wavelengths split at the third beam splitter 238 on the return path from
the optical beam of the first through third wavelengths on the outgoing
path, and a multi lens 246 provided between the third beam splitter 238
and the photosensor 245, for condensing optical beams of the first
through third wavelengths split at the third beam splitter 238 onto the
photoreception face of a photodetector or the like of the photosensor
245, and also providing astigmatism for detecting focus error signals or
the like.

[0362]The first light source 231 has a first emitting unit for emitting an
optical beam of a first wavelength around 405 nm onto the first optical
disc 11. The second light source 232 has a second emitting unit for
emitting an optical beam of a second wavelength around 655 nm onto the
second optical disc 12. The third light source 233 has a third emitting
unit for emitting an optical beam of a third wavelength around 785 nm
onto the third optical disc 13. Note that while the first through third
emitting units are configured disposed at individual light sources 231,
232, and 233, the invention is not restricted to this, and an arrangement
may be made wherein two emitting units of the first through third
emitting units are disposed at one light source and the remaining
emitting unit is disposed at another light source, or wherein the first
through third emitting units are disposed so as to form a light source at
generally the same position.

[0363]The object lens 234 condenses the input optical beams of the first
through third wavelengths into the signal recording face of the optical
disc 2. The object lens 234 is movably held by an object lens driving
mechanism such as an unshown biaxial actuator or the like. The object
lens 234 is driven along two axes, one in the direction toward/away from
the optical disc 2, and the other in the radial direction of the optical
disc 2, by being moved by a biaxial actuator or the like based on the
tracking error signals and focus error signals generated from the FR
signals of the return light from the optical disc 2 that has been
detected at the photosensor 245. The object lens 234 condenses optical
beams emitted from the first through third emitting units such that the
optical beams are always focused on the signal recording face of optical
disc 2, and also causes the focused beam to track a recording track
formed on the signal recording face of the optical disc 2. Note that an
arrangement is made wherein, as described later, in the case of a
diffraction unit 250 being provided on an optical element (diffraction
optical element 235B) separate from the object lens (see FIG. 58), the
later-described diffraction optical element 235B is held by a lens holder
of the object lens driving mechanism where the object lens 234B is held
so as to be integral with the object lens 234B enables the
later-described advantages of the diffraction unit 250 provided to the
diffraction optical element 235B to be suitably manifested at the time of
field shift of the object lens 234B such as movement in the tracking
direction.

[0364]Also, with the object lens 234, as one face thereof, for example,
the diffraction unit 250 made up of multiple diffraction regions is
provided on the incident side face, and according to this diffraction
unit 250, each of the optical beams of the first through third
wavelengths passing through each of the multiple diffraction regions is
diffracted so as to become a predetermined order, thereby entering the
object lens 234 as optical beams in a diffused state or converged state
having a predetermined divergent angle, and accordingly, the single
object lens 234 can be used to perform suitable condensing of the optical
beams of the first through third wavelengths such that spherical
aberration does not occur at the signal recording face of the three types
of optical discs corresponding to the optical beams of the first through
third wavelengths. The object lens 234 including the diffraction unit 250
serves as a condensation optical device for appropriately performing
condensation such that no spherical aberration occurs at the signal
recording face of the three types of optical discs corresponding to the
optical beams of the three different wavelengths by a diffraction
structure being formed which generates diffraction power with a lens face
shape for generating diffraction power serving as reference. Also, thus,
the object lens 234 has both of a refraction element function and a
diffraction element function, i.e., has both of a refraction function
according to a lens curved surface, and a diffraction function according
to the diffraction unit 250 provided on one face.

[0365]Now, in order to describe the diffraction function of the
diffraction unit 250 conceptually, as described later, description will
be made regarding a case wherein the diffraction unit 250 is provided on
the diffraction optical element 235B separate from the object lens 234B
having refractive power (see FIG. 58) as an example. The diffraction
optical element 235B, which is employed along with the object lens 234B
having a refraction function alone as described later, having the
diffraction unit 250 performs, for example, as shown in FIG. 38A,
diffraction of the first wavelength optical beam BB0 which has
transmitted the diffraction unit 250 so as to become +1st order
diffracted beam BB1 and inputs to the object lens 234B, i.e., as a beam
in a diffused state having a predetermined divergent angle, thereby
appropriately condensing on the signal recording face of the first
optical disc 11, as shown in FIG. 38B, performs diffraction of the second
wavelength optical beam BD0 which has transmitted the diffraction unit
250 so as to become -1st order diffracted beam BD1 and inputs to the
object lens 234B, i.e., as a beam in a converged state having a
predetermined divergent angle, thereby appropriately condensing on the
signal recording face of the second optical disc 12, as shown in FIG.
38C, and performs diffraction of the third wavelength optical beam BC0
which has transmitted the diffraction unit 250 so as to become -2nd order
diffracted beam BC1 and inputs to the object lens 234B, i.e., as a beam
in a converged state having a predetermined divergent angle, thereby
appropriately condensing on the signal recording face of the third
optical disc 13, whereby suitable condensation can be performed such that
no spherical aberration occurs at the signal recording face of the three
types of optical discs, with a single object lens 234B. While description
has been made here with an example wherein optical beams of the same
wavelength are made to be diffracted beams of the same diffraction order
at the multiple diffraction regions of the diffraction unit 250, with
reference to FIG. 38, the diffraction unit 250 configuring the optical
pickup 3 to which the present invention is applied enables diffraction
order corresponding to each wavelength to be set for each region as
described later, so as to perform suitable aperture restriction, and
further reduce spherical aberration. Description has been made so far
regarding a case wherein the diffraction unit 250 is provided on an
optical element separate from the object lens for the sake of description
as an example, but the diffraction unit 250 provided integral with one
face of the object lens 234 described here also has the same function by
providing diffraction power according to the diffraction structure
thereof, and the diffraction power of the diffraction unit 250, and the
refractive power according to a lens curved face serving as the reference
of the object lens 234 enable the optical beams of each wavelength to be
condensed appropriately on the signal recording face of the corresponding
optical disc such that no spherical aberration occurs.

[0366]In the above and following description of diffraction orders, an
order of diffraction which draws closer to the optical axis side in the
direction of progression with regard to an input optical beam is a
positive order, and an order of diffraction which separates from the
optical axis in the direction of progression is a negative order. In
other words, an order which diffracts toward the optical axis of the
input optical beam is a positive order.

[0367]Specifically, as shown in FIGS. 39A and 39B, the diffraction unit
250 provided at the incident side face of the object lens 234 has a
generally-circular first diffraction region 251 provided on the innermost
portion (hereinafter also referred to as "inner ring zone"), a
ring-shaped second diffraction region 252 provided on the outer side of
the first diffraction region 251 (hereinafter also referred to as "middle
ring zone"), and a ring-shaped third diffraction region 253 provided on
the outer side of the second diffraction region 252 (hereinafter also
referred to as "outer ring zone").

[0368]The first diffraction region 251 which is an inner ring zone has a
first diffraction structure formed having a ring shape with a
predetermined depth, and diffracts the optical beam of the first
wavelength that is transmitted therethrough such that diffracted light of
an order which forms an appropriate spot on the signal recording face of
the first optical disc via the object lens 234 is dominant, i.e., such
that maximum diffraction efficiency is manifested regarding diffracted
light of other orders.

[0369]Also, the first diffraction region 251 diffracts the optical beam of
the second wavelength that is transmitted therethrough such that
diffracted light of an order which forms an appropriate spot on the
signal recording face of the second optical disc via the object lens 234
is dominant, i.e., such that maximum diffraction efficiency is manifested
regarding diffracted light of other orders, by way of the first
diffraction structure.

[0370]The first diffraction region 251 diffracts the optical beam of the
third wavelength that is transmitted therethrough such that diffracted
light of an order which forms an appropriate spot on the signal recording
face of the third optical disc via the object lens 234 is dominant, i.e.,
such that maximum diffraction efficiency is manifested regarding
diffracted light of other orders, by way of the first diffraction
structure.

[0371]Thus, the first diffraction region 251 has a diffraction structure
formed whereby diffracted light of a predetermined order is dominant in
the optical beam of each wavelength, thereby enabling correction and
reduction of spherical aberration at the time of optical beams of each
wavelength that have passed through the first diffraction region 251 and
become diffracted light of a predetermined order being condensed on the
signal recording face of the respective optical discs by the object lens
234.

[0372]Note that regarding the first diffraction region 251, and also the
second and third diffraction regions 252 and 253 described in detail
later, description is made in the above and below with the understanding
that transmitted light, i.e., light of zero order, is included in the
diffracted light of a predetermined order selected so as to be dominant
with regard to the optical beam of each wavelength.

[0373]Specifically, as shown in FIGS. 39 and 40A, the first diffraction
region 251 is formed with the cross-sectional form of ring shapes being
formed as to the reference face of ring shapes centered on the optical
axis, and with step shapes (hereafter referred to as "multiple steps of
step shapes") of a predetermined number of steps S (S is assumed to be a
positive integer) of a predetermined depth (hereinafter also referred to
as "groove depth") d being formed consecutively in the radial direction.
Note that the cross-sectional form of the ring shapes in this diffraction
structure means the cross-sectional form of the rings taken along a plane
including the radial direction of the rings, i.e., a plane orthogonal to
the tangential direction of the rings.

[0374]Also, this reference face means the face shape of the incident side
face required as a refraction element function of the object lens 234.
With the first diffraction region 251, in reality, as shown in FIG. 39A,
with the face shape of the incident side face required as a refraction
element function of the object lens 234 as a reference face, as to this
reference face, there is formed a face shape such as a combination of a
ring zone form face shape and staircase form face shape making up a
diffraction structure having a diffraction function such as shown in FIG.
40A, but in FIG. 39A through 39C and later-described FIG. 47, a
diffraction structure shape alone as to the reference face thereof is
illustrated for the sake of description, and also in the following
description, the shape as to the reference face will be described. Note
that in the case of providing the diffraction unit 250 in an optical
element (later-described diffraction optical element 235B) separate from
the object lens, the shapes illustrated in FIGS. 39A through 39C become
the cross-sectional shape of the relevant diffraction optical element
235B. Also, the diffraction structure illustrated in FIG. 39 and so forth
is actually formed with minute dimensions such as described later, and
FIG. 39 and so forth illustrate enlarged cross sections.

[0375]Also, the diffraction structure having the staircase form with a
predetermined number of steps S is a structure in which a staircase form
having first through S steps, each of which have generally the same
depth, continuing in the radial direction, which can be rephrased as
saying that the structure has first through S+1'th diffraction faces
formed with generally the same interval in the optical axis direction.
Also, the predetermined depth d in the diffraction structure means the
length along the optical axis between the diffraction face of the S+1'th
diffraction face which is formed at the side of the staircase form
closest to the surface (i.e., the highest step, which is the shallowest
position) and diffraction face of the first diffraction face which is
formed at the side of the staircase form closest to the optical element
(i.e., the lowest step, which is the deepest position). Note that while a
structure has been illustrated in FIG. 40A wherein the steps of each
stepped portion of the staircase shape are formed such that the closer to
the inner side in the radial direction, the closer to the surface side
the steps are formed, this is because a later-described diffraction order
is selected as the maximum diffraction efficiency order in an inner ring
zone. Also, in FIGS. 40B and 40C, and later-described FIG. 47, examples
are illustrated wherein similar to an inner ring zone, the saw-tooth
slopes of the protrusions and recesses or the stepped portions of the
staircase shape are formed such that the closer to the inner side in the
radial direction, the closer to the surface side the saw-tooth slopes of
the protrusions and recesses or the stepped portions of the staircase
shape are formed, the present invention is not restricted to this, the
formation direction of the blazed shape or staircase shape is set
according to the selected diffraction order. Ro in FIG. 40A through 40C
indicates the direction toward the outer side in the radial direction of
a ring zone, i.e., the direction separated from the optical axis.

[0376]In the first diffraction structure and the later-described second
and third diffraction structures formed at the first diffraction region
251, the groove depth d and number of steps S are determined taking into
consideration the dominant diffraction order and diffraction efficiency.
Also, as shown in FIG. 40A, the groove width of each step portion (the
radial-direction dimension of each step portion of the staircase form) is
such that the steps are formed with equal width within one staircase
form, while looking at the different staircase forms formed continuously
in the radial direction, the value of the step width is smaller as
staircase forms further away from the optical axis. Note that description
has been made here assuming that such an arrangement is employed as
described above, but the groove width of each step portion is such that
while looking at the different staircase forms formed continuously in the
radial direction, the value of the step width is grater as staircase
forms further away from the optical axis in some cases. This point is
also true for FIGS. 40B and 40C. Note that the groove widths are
determined based on phase difference obtained at the diffraction regions
formed with the groove widths, such that the spot condensed on the signal
recording face of the optical disc is optimal.

[0377]For example, the diffraction structure of the first diffraction
region 251 is, as shown in FIG. 40A, a diffraction structure having a
staircase portion including first through fourth steps 251s1, 251s2,
251s3, and 251s4, formed continuously in the radial direction, wherein
the number of steps is 4 (S=4), and the depth of each step is generally
the same depth (d/4), and first through fifth diffraction faces 251f1,
251f2, 251f3, 251f4, and 251f5 formed at the same intervals of d/4 in the
optical axis direction.

[0378]Also, in a case wherein the first diffraction region 251 diffracts
the optical beam of the first wavelength which is transmitted
therethrough such that diffracted light of the k1i'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, diffracts
the optical beam of the second wavelength which is transmitted
therethrough such that diffracted light of the k2i'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, and
diffracts the optical beam of the third wavelength which is transmitted
therethrough such that diffracted light of the k3i'th order is dominant,
an arrangement is made so as to have the relation of
k1i≧k2i>k3i.

[0379]Thus, according to the arrangement wherein diffracted light is
generated so as to have the relation of k1i≧k2i≧k3i, the
first diffraction region 251 makes not only the diffracted light of an
order whereby spherical aberration can be reduced appropriately dominant
but also the relation between operating distance and focal distance
changed to most appropriate state, ensuring the operating distance in the
case of employing the third wavelength λ3 makes the focal distance
long as to the first wavelength λ1, whereby problems can be
prevented such as the lens diameter of the object lens and the optical
pickup overall increasing in size, and also aberration can be reduced
while ensuring diffraction efficiency.

[0380]Now, description will be made regarding a method for selecting the
optimal diffraction order including the reason why an arrangement is made
so as to have the relation of k1i≧k2i≧k3i with the first
diffraction region 251 based on the following first through fourth
perspectives. In other words, with the first diffraction region 251, as
the first perspective there is a need to reduce spherical aberration at
each wavelength, as the second perspective there is a need to optimize
operating distance and focal distance at each wavelength, and as the
third and fourth perspectives there is a need to employ the structure
which is advantageous in manufacturing and can be readily manufactured,
and consequently, from these perspectives, the diffraction orders k1i,
k2i, and k3i have been selected as diffraction orders with maximum
diffraction efficiency, and description will be made below regarding this
point.

[0381]First, the first perspective will be described. As the first
perspective, there is a need to employ an order whereby the spherical
aberration of the corresponding optical disc can be corrected at the time
of condensing light with the object lens 234 as the diffraction order
with the first diffraction region 251 which is an inner ring zone. In
general, in a case wherein material dispersion is ignored at a region
having a function such as the first diffraction region 251, it is known
that satisfying the conditional expression

(λ1×k1x-λ2×k2x)/(t1-t2)≈(λ1×k1-
x-λ3×k3x)/(t1-t3) (1)

[0382]where λ1 is the first wavelength (nm), λ2 is the second
wavelength (nm), λ3 is the third wavelength (nm), k1i is the
diffraction order where an optical beam of the first wavelength is
selected, k2i is the diffraction order where an optical beam of the
second wavelength is selected, k3i is the diffraction order where an
optical beam of the third wavelength is selected, t1 is the thickness
(mm) of the first protective layer of the first optical disc, t2 is the
thickness (mm) of the first protective layer of the second optical disc,
t3 is the thickness (mm) of the first protective layer of the third
optical disc, and x=i for the inner ring zone in k1x, k2x, and k3x in
this conditional expression,

[0383]is a condition whereby spherical aberration on the signal recording
face of each optical disc at each wavelength can be corrected and
reduced.

[0384]In the first diffraction region 251 which is the above-described
inner ring zone, when λ1=405 (nm), λ2=655 (nm), λ3=785
(nm), t1=0.1 (mm), t2=0.6 (mm), and t3=1.1 (mm), then k1i=+1, k2i=-1, and
k3i=-2, each hold, thereby satisfying the conditional expression, and it
has been confirmed that spherical aberration can be reduced. This can be
restated in other words that when plotting points Pλ1, Pλ2,
and Pλ3 in the graph in FIG. 41 wherein the horizontal axis
represents a value calculated by wavelength×diffraction order (nm)
and the vertical axis represents the thickness (mm) of the protective
layer, the points are on a generally straight design line, meaning that
spherical aberration on the signal recording face of each optical disc at
each wavelength can be corrected and reduced, but actually when plotting
the respective points Pλ1, Pλ2, and Pλ3 under the
following conditions, the respective points are positioned on a generally
straight design line, meaning that spherical aberration can be corrected
and reduced. Specifically, the object lens 234 has the material of which
it is configured, and the face shape at the input and output sides,
determined with the line L21 in FIG. 41 as the design line, with the
inclination of the design line approximating the inclination of the line
connecting Pλ1 and Pλ2 calculated by
(t1-t2)/(λ1×k1i-λ2×k2i) or the inclination of the
line connecting Pλ1 and Pλ3 calculated by
(t1-t3)/(λ1×k1i-λ3×k3i), or determined taking
into consideration the inclination of these lines and other design
conditions.

[0385]Note that while in FIG. 41 Pλ3 deviates slightly upwards from
the line L21, spherical aberration can be corrected in a sure manner by
inputting the incident light to the object lens 234 where the diffraction
unit 250 is provided, as a divergent ray. That is to say, a divergent ray
is input to the object lens 234, whereby the same result as that in the
case of the apparent thickness of the protective layer being thickened
can be obtained. Note that, as described later, in the case of providing
the diffraction unit 250 in an optical element (diffraction optical
element 235B, see FIG. 58) separate from the object lens, spherical
aberration can be corrected in a sure manner by inputting the incident
light to the one of the object lens 234B and diffraction optical element
235B which is closer to the emitting units, which is, for example, the
diffraction optical element 235 in FIG. 58, as a divergent ray.

[0386]Description will be made regarding this point with reference to FIG.
42 illustrating the concept of this correction. Specifically, the optical
beams of the second and third wavelengths λ2 and λ3 are input
to the object lens 234 as minimal divergent rays, thereby shifting plots
Pλ2' and Pλ3' indicating the second and third wavelengths
upward as to the plots Pλ2 and Pλ3 according to the apparent
thickness of the protective layer, as shown in FIG. 42. As shown in FIG.
42, the magnification of a divergent ray is adjusted appropriately,
whereby these three points Pλ1, Pλ2', and Pλ3' can be
positioned on one straight line L21' completely, and spherical aberration
due to difference of protective layer thickness and so forth can be fully
corrected. At this time, the straight line L21' where the plots
Pλ1, Pλ2', and Pλ3' are positioned are taken as a
design line.

[0387]Note here that, for example, an arrangement may be made wherein only
the optical beam of the third wavelength λ3 is input as a
convergent ray, and is shifted downward to position the respective plots
on a straight line, thereby correcting spherical aberration, but
employing a convergent ray shortens the operating distance, which is
undesirable in some cases, and accordingly, it is desirable to employ a
divergent ray as described above. Further, when compatibility of three
wavelengths is taken into consideration, it is advantageous to input a
divergent ray to the object lens with the second and third wavelengths
from the perspective wherein appropriate return magnification can be
ensured.

[0388]Also, when the plots Pλ1, Pλ2, and Pλ3 having
close connection with the above-mentioned relational expression,
described with reference to FIG. 41 are taken into consideration, if the
absolute values of the respective orders k1i, k2i, and k3i are within a
range of around 3rd order, there is a need to satisfy the following
relational expression (2A) or (2B).

k1i≦k2i≦k3i (2A)

k1i≧k2i≧k3i (2B)

[0389]Next, the second perspective will be described. As the second
perspective, there is a need to employ an order whereby focal distance f1
as to the first wavelength λ1 can be reduced while maintaining the
operating distance WD3 large in the case of employing the third
wavelength λ3. In general, extending the focal distance f extends
the operating distance. The focal distance f1 as to the first wavelength
λ1 needs to be reduced, and the focal distance f3 as to the third
wavelength λ3 needs to be increased. Now, it is desirable to
suppress the focal distance f1 as to the first wavelength λ1 to 2.2
mm or shorter. Also, there is a need to ensure the operating distance of
around 0.4 mm or longer in the case of employing the third wavelength
λ3. In order to realize these, if we say that f1=2.2 mm, and
incidence to the object lens 234 is infinite incidence, i.e., parallel
light incidence, f3 needs to be around 2.5 mm or longer. With the
material of the object lens made from plastics corresponding to the
above-mentioned three wavelengths λ1, λ2, and λ3,
dispersion is great, but let us say that this is ignored here, and an
approximate value is calculated.

[0390]The object lens 234 has refractive power according to a lens curved
face, and diffraction power according to the diffraction unit 250
provided on one face. It has been known that focal distance Fdif
according to diffraction of the diffraction unit 250 of the object lens
234 can be calculated in accordance with the following Expression (3). In
Expression (3), λ0 is a manufacturing wavelength, and now, let us
say that λ0=λ1. Also, C1 is a value called a phase
difference function coefficient, which is a coefficient for stipulating a
phase difference shape provided by a diffraction structure (diffraction
grating), and is a variable value depending on the value of λ0.
Also, in Expression (3), k represents a diffraction order selected by
each of the wavelengths λ1, λ2, and λ3, and
specifically is k1, k2, or k3.

λλ ##EQU00001##

[0391]In Expression (3), with the coefficient C1, if we say that
λ0=λ1, the absolute value thereof is not smaller than
1×10-2, the amount of pitches increases, and consequently,
formation becomes impossible. Also, if we say that the focal distance
according to the refractive power of a lens curved face is fr, focal
distance fall of the refraction and diffraction overall of the
object lens is calculated according to the relation of Expression (4)
using the above-mentioned focal distance fdif according to
diffraction, and this fr.

##EQU00002##

[0392]FIG. 43 illustrates change in the value of the focal distance f3
when changing k1 and k3 based on such Expressions (3) and (4). In FIG.
43, the horizontal axis represents the order k3, and the vertical axis
represents the focal distance f3 as to the third wavelength λ3, and
curves LM3, LM2, LM1, LP0, LP1, LP2, and LP3 represent curves connecting
plotted changes in the focal distance f3 along with change in k3i in the
case of the corresponding orders k1i being -3rd order, -2nd order, -1st
order, zero-order, 1st order, 2nd order, and 3rd order. Note that FIG. 43
illustrates calculation results assuming that the coefficient C1 is
1×10-2 which is the maximum, and fall1 representing the
overall focal distance fall calculated by Expression (4) of the
first wavelength λ1 is fall1=2.2 (mm). The diffraction order
has thus been described above, but actually, geometrical optics can be
applied to the inner ring zone portion alone, and the properties such as
the focal distance and so forth are determined with the inner ring zone
portion, so the above-mentioned k1 through k3 correspond to k1i through
k3i, and in other words, the above-mentioned relation of k1 through k3
also has the relation where k1 through k3 are substituted for k1i through
k3i respectively. According to FIG. 43, in order to set f3 to 2.5 mm or
longer, the relation of the following Expression (5A) holds. Accordingly,
in order to ensure appropriate focal distance and operating distance, it
is necessary to have the relation of the following Expression (5B) from
the above-mentioned relation of Expression (2B).

k1i>k3i (5A)

k1i≧k2i>k3i (5B)

[0393]Further, from an perspective wherein this Expression (5B) and a
later-described restriction that a diffraction order to be employed is
equal to or smaller than around 3, each of combinations of (k1i,
k3i)=(-2, -3), (-1, -2), (-1, -3), (0, -2), (0, -3), (1, -2), (1, -3),
(2, -1), (2, -2), (2, -3), (3, 0), (3, -1), (3, -2), and (3, -3) is a
suitable combination from the above-mentioned perspective. At this time,
k2i determined so as to satisfy Expression (5B) is employed. Note that,
strictly, the relation in FIG. 43 is changed with the value of f1 and
material dispersion, and further, the target value of f3 deteriorates by
deteriorating f1, or changing incident magnification to the object lens
to a divergent ray, but the above-mentioned choices of diffraction orders
are suitable.

[0394]Next, description will be made regarding the third perspective. As
the third perspective, the configuration needs to be advantageous in
manufacturing. In a case wherein a diffraction order to be selected is
too great, the steps of the diffraction structure to be formed, and the
depth of blaze become deep. Further, when the depth of the diffraction
structure becomes deep, there is a possibility that formation precision
deteriorates, and also there is a possibility that a problem occurs
wherein an optical path length enhancement effect due to change in
temperature increases, and temperature diffraction efficiency properties
deteriorate. Also, there is a problem wherein deterioration in formation
precision leads to deterioration in diffraction efficiency. It is
desirable and common from such reasons to select a diffraction order up
to around 3rd through 4th. Accordingly, with the above-mentioned second
perspective, study has been made employing a diffraction order up to 3rd.

[0395]Next, description will be made regarding the fourth perspective. As
the fourth perspective, though similar to the third perspective, the
diffraction structure needs to be able to be manufactured. When
performing a diffraction efficiency calculation described in a
later-described section of "Depth and shape of diffraction structure and
diffraction efficiency", the depth d needs to be equal to or smaller than
a suitable size, and the diffraction structure needs to be formed with
this depth. Further, the depth d needs to be equal to or smaller than at
least 15 μm.

[0396]From the above-mentioned first through fourth perspectives, the
first diffraction region 251 which is an inner ring zone is configured so
as to generate each diffracted light having relation of
k1i≧k2i>k3i.

[0397]Further, the first diffraction region 251 is configured such that,
of the diffraction orders k1i, k2i, and k3i of each wavelength of which
the diffraction efficiency is the maximum, k1i and k3i have any of the
following relations.

[0399]Also, from the first through fourth perspectives, specifically, as
described later, the optimal configuration example is a case wherein
(k1i, k2i, k3i)=(1, -1, -2), (0, -1, -2), (1, -2, -3) or (0, -2, -3).
Now, when the diffraction orders k1i, k2i, and k3i are selected as above,
the number of steps S and groove depth d selected at the time of
diffraction efficiency and so forth being taken into consideration are
shown in I1 through I4 in Table 6. Also, in Table 6, additionally, with
the relation of the plots Pλ1, Pλ2, and Pλ3, and design
line L described with reference to FIG. 41, a later-described deviation
amount Δ from the design line L of the plot Pλ3 indicating
the third wavelength is shown in Table 6. That is to say, as shown in
later-described FIG. 48, when setting a line connecting the plots
Pλ1 and Pλ2 (hereafter, referred to as "spherical aberration
correction line"), this deviation amount Δ indicates the distance
deviated in the vertical axis direction (direction indicating protective
layer thickness) from the plot Pλ3 toward the spherical aberration
correction line thereof. Here, in the case of the deviation amount
Δ=0, this indicates that the respective points Pλ1,
Pλ2, and Pλ3 are on a straight line completely. Also, in the
case of the deviation amount Δ is positive, this indicates that the
plot Pλ3 is positioned lower than the spherical aberration
correction line, and in the case of the deviation amount A is negative,
this indicates that the plot Pλ3 is positioned upper than the
spherical aberration correction line. Note here that in FIG. 41
illustrating the first embodiment of an inner ring zone, it is difficult
to illustrate this deviation amount Δ from the features of inner
ring zones, so description has been made regarding this deviation amount
Δ using FIG. 48 employed for the first embodiment of an middle ring
zone, but let us say that the definition regarding this deviation amount
Δ is true for both inner ring zones and middle ring zones. As shown
in Table 6, in any example, diffraction efficiency is sufficiently
ensured, and the deviation amount Δ is also sufficiently small, and
accordingly, a suitable diffraction order can be confirmed even if
spherical aberration correction is taken into consideration.

[0400]Next, description will be made regarding "Calculation of depth and
shape of diffraction structure and diffraction efficiency" with the first
diffraction region 251 and so forth with reference to a specific
embodiment. Now, a diffraction face design example such that the
diffracted light of each order described above is taken as the maximum
diffracted light will be shown as the inner ring zone according to the
first embodiment with reference to FIG. 44. Note that the diffraction
amount (diffraction efficiency) of the selected diffraction order
fluctuates depending on groove depth such as shown in FIG. 44, so setting
suitable groove depth enables the diffraction efficiency of the selected
diffraction order at each wavelength to be increased up to a desired
level.

[0401]Specifically, FIGS. 44A through 44C illustrate change in diffraction
efficiency as to the groove depth d when assuming that the diffraction
structure is the staircase form of the number of steps S=4, and (k1i,
k2i, k3i)=(+1, -1, -2). FIG. 44A is a diagram illustrating change in
diffraction efficiency of +1st order diffracted light of the optical beam
of the first wavelength, FIG. 44B is a diagram illustrating change in
diffraction efficiency of -1st order diffracted light of the optical beam
of the second wavelength, and is also a diagram illustrating change in
diffraction efficiency of -2nd order diffracted light serving as unwanted
light as described later, and FIG. 44C is a diagram illustrating change
in diffraction efficiency of -2nd order diffracted light of the optical
beam of the third wavelength, and is also a diagram illustrating change
in diffraction efficiency of +3rd order diffracted light serving as
unwanted light as described later. In FIGS. 44A through 44C, the
horizontal axis represents groove depth (nm), and the vertical axis
represents diffraction efficiency (light intensity). If we say that the
diffraction efficiency of k1i is eff1, the diffraction efficiency of k2i
is eff2, and the diffraction efficiency of k3i is eff3, the position of
the groove depth d=3800 (nm) shown in the horizontal axis has sufficient
diffraction efficiency. Specifically, as shown in FIG. 44A eff1=0.81, as
shown in FIG. 44B eff2=0.62, and as shown in FIG. 44C eff3=0.57, which
have sufficient diffraction efficiency. As shown in FIGS. 44A through
44C, the relation between diffraction efficiency and groove depth
fluctuates depending on the number of steps, so there is a need to select
a suitable number of steps, but the number of steps S=4 is employed here,
as described above.

[0402]With the first diffraction region 251, the inner ring zone region is
configured of a step structure (diffraction structure of staircase form),
which is a configuration suitable for deviating the diffraction
efficiency of unwanted light generated at this diffraction region from
the diffraction efficiency eff1, eff2, and eff3 of regular light. Now,
let us say that the term "regular light" means diffracted light of the
diffraction orders k1i, k2i, and k3i thus selected, i.e., the diffracted
light of a diffraction order of which the diffraction efficiency becomes
the maximum, and the term "unwanted light" means the diffracted light of
a diffraction order of which the diffraction efficiency becomes the
second largest diffraction efficiency. Note that in FIGS. 44A through
44C, and later-described FIGS. 45A through 45C and 54A through 54C, LM
represents change in the diffraction efficiency of the diffracted light
of the diffraction order of which the diffraction efficiency becomes the
maximum, and LF represents change in the diffraction efficiency of the
diffracted light of the diffraction order serving as unwanted light
described here.

[0403]Description will be made wherein with the first diffraction region
251, the diffraction structure having the staircase form is formed,
whereby the influence of unwanted light can be reduced. In order to
compare to this FIGS. 44A through 44C, the diffraction efficiency in the
case of this inner ring zone being formed as a blazed shape is
illustrated in FIGS. 45A through 45C as a reference example. FIGS. 45A
through 45C illustrate change in the diffraction efficiency as to the
groove depth d when assuming that the diffraction structure is formed as
a blazed shape of the number of steps S=∞, and (k1i, k2i, k3i)=(+1,
+1, +1). FIG. 45A is a diagram illustrating change in the diffraction
efficiency of the +1st order diffracted light of the optical beam of the
first wavelength, FIG. 45B is a diagram illustrating change in the
diffraction efficiency of the +1st order diffracted light of the optical
beam of the second wavelength, and also illustrating change in the
diffraction efficiency of the zero-order light serving as unwanted light,
and FIG. 45C is a diagram illustrating change in the diffraction
efficiency of the +1st order diffracted light of the optical beam of the
third wavelength, and also illustrating change in the diffraction
efficiency of the zero-order light serving as unwanted light. In FIG. 45A
through 45C, the horizontal axis represents groove depth (nm), and the
vertical axis represents diffraction efficiency (light intensity). As
shown in FIGS. 45A through 45C, in the case of the second and third
wavelengths, the zero-order light has efficiency as unwanted light. With
each optical beam of adjacent diffraction orders such as the zero-order
light and 1st order light, diffraction angles have few differences. Thus,
when the regular light which is either optical beam of the selected
diffraction orders k2i and k3i is condensed on the corresponding optical
disc so as to be in a focused state, unwanted light is also condensed in
a blurring state. Subsequently, this unwanted light is also reflected at
the optical disc, and the reflection light of the unwanted light is
irradiated on a photoreceptor portion, which has adverse influence upon a
signal obtained at the photoreceptor portion, and there is a possibility
that jittering or the like will deteriorate. Further, there is a
possibility that this unwanted light leads to a problem wherein in the
case of defocus occurring, the influence thereof becomes great. As shown
in FIGS. 44A through 44C described above, the diffraction structure
having the staircase form is formed, whereby the diffraction efficiency
of unwanted light can be reduced as compared to the case shown in FIGS.
45A through 45C.

[0404]That is to say, in a case wherein an inner ring zone portion such as
the first diffraction region 251 is formed in a staircase form, a
structure can be realized whereby the quantity of the diffracted light of
unwanted light is suppressed. With the diffraction structure having the
staircase form, groove depth which deteriorates the efficiency of
unwanted light can be selected, and even if unwanted light efficiency
becomes high efficiency, the order serving as regular light and the order
serving as unwanted light differ greatly, whereby unwanted light can be
prevented from condensing at the time of focus. Specifically, as shown in
FIG. 44B, the unwanted light efficiency according to the second
wavelength can be suppressed to around 5% which does not contribute.
Also, as shown in FIG. 44C, the regular light according to the third
wavelength is -2nd order light, but unwanted light is +3rd order light,
and with this -2nd order light and +3rd order light, diffraction angles
differs greatly, so unwanted light is defocused greatly in the case of
regular light being focused, and accordingly, there is no bad influence
due to unwanted light being input to the photoreceptor portion. In other
words, a so-called step structure such as a staircase form is a structure
suitable for deviating the diffraction efficiency of regular light from
the diffraction efficiency of the diffracted light of the adjacent orders
as compared to a blazed form or the like.

[0405]Next, description will be made regarding "pitch design" according to
the first diffraction region 251 and so forth. With the pitch design of
the diffraction structure, if we say that a phase to be provided by a
diffraction unit (diffraction face) having a predetermined diffraction
structure is φ, the phase thereof can be represented as the following
Expression (6) using the phase difference function coefficient Cn. Note
that in Expression (6), k represents the diffraction order to be selected
at the respective wavelengths λ1, λ2, and λ3, and
specifically, represents k1, k2, and k3, r represents a position in the
radial direction, and λ0 represents a design wavelength. Now, let
us say that in the case of λ0 employed for pitch design,
calculation is performed assuming k=1.

φ λ ##EQU00003##

[0406]In Expression (6), the value of φ can be obtained uniquely at
the time of lens design. On the other hand, φ represents the phase of
the design wavelength λ0, and with φ' obtained from the
relational expression of φ'=φ-nλ0, and the phase obtained
by this φ, the given influence thereof is completely the same. In
other words, φ' obtained from the above-mentioned relational
expression is, as shown in FIG. 46B, is a remainder in the case of
dividing φ by λ0 such as shown in FIG. 46A for example, i.e., a
value obtained by a so-called remainder calculation. This φ' can be
referred to as phase amount to be provided for determining the pitches of
the actual diffraction structure. The actual diffraction structure
pitches are determined from this φ', and specifically, as shown in
FIG. 46C, are determined so as to be along with the shape of this φ'.
Note that the horizontal axes in FIG. 46A through 46C represent a
position in the radial direction, the vertical axis in FIG. 46A
represents necessary phase amount φ for each position thereof, the
vertical axis in FIG. 46B represents granting phase amount φ'
obtained by remainder calculation for each position thereof, and the
vertical axis in FIG. 46C represents groove depth d. Here, in FIG. 46C,
after pitches are determined, a blazed shape is illustrated, but in the
case of employing a staircase form such as the above-mentioned first
diffraction region 251 or the like, the blazed slope portion shown in
FIG. 46C is formed in a staircase form of a predetermined number of steps
S.

[0407]Note that description has been made above assuming that of the
diffraction structure provided in the first diffraction region 251, the
cross-sectional shape including the radial direction and optical axis
direction thereof has, as shown in FIG. 40A, the diffraction structure of
multiple staircase forms formed with predetermined height and
predetermined width set generally with an equal interval within one
staircase portion, but the present invention is not restricted to this,
an non-cyclical step form may be formed such that the height and/or width
of a staircase form serving as reference is finely adjusted based on an
acquisition target phase such as shown in FIG. 46B. Further, a form
determined by phase design may be formed so as to provide predetermined
phase difference to the optical beam of a predetermined wavelength, i.e.,
the cross-sectional shape may not be formed of only a straight line
parallel to a horizontal line indicating a plane serving as reference,
and a perpendicular line, but may be formed so as to be an non-cyclical
form including a straight line (sloping surface) inclined as to that
straight line, curve (curved surface), or the like. This point is true
for a later-described second diffraction region 252.

[0408]A second diffraction region 252 which is a middle ring zone, where a
second diffraction structure is formed, which is a structure different
from the first diffraction structure having a ring zone shape and
predetermined depth, diffracts the optical beam of the first wavelength
that is transmitted therethrough such that diffracted light of an order
which forms an appropriate spot on the signal recording face of the first
optical disc via the object lens 234 is dominant, i.e., such that maximum
diffraction efficiency is manifested regarding diffracted light of other
orders.

[0409]Also, the second diffraction region 252 diffracts the optical beam
of the second wavelength that is transmitted therethrough such that
diffracted light of an order which forms an appropriate spot on the
signal recording face of the second optical disc via the object lens 234
is dominant, i.e., such that maximum diffraction efficiency is manifested
regarding diffracted light of other orders, by way of the second
diffraction structure.

[0410]Also, the second diffraction region 252 diffracts the optical beam
of the third wavelength that is transmitted therethrough such that
diffracted light of orders other than an order which forms an appropriate
spot on the signal recording face of the third optical disc via the
object lens 234 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders, by
way of the second diffraction structure. If it puts in another way
regarding this point, in light of later-described flaring operation and
so forth, the second diffraction region 252 diffracts the optical beam of
the third wavelength that is transmitted therethrough such that
diffracted light of an order which forms no appropriate spot on the
signal recording face of the third optical disc via the object lens 234
is dominant, by way of the second diffraction structure. Note that the
second diffraction region 252 can sufficiently reduce diffraction
efficiency diffracted light of an order which forms an appropriate spot
on the signal recording face of the third optical disc via the object
lens 234 for the optical beam of the third wavelength that is transmitted
therethrough, by way of the second diffraction structure.

[0411]Thus, with the second diffraction region 252, there is formed a
diffraction structure suitable for the diffracted light of a
predetermined order being dominant as to the optical beam of the
above-mentioned respective wavelengths, thereby enabling spherical
aberration to be corrected and reduced at the time of the optical beams
of the first and second wavelengths serving as the diffracted light of a
predetermined order that is transmitted therethrough being condensed on
the signal recording face of the corresponding optical disc via the
object lens 234.

[0412]Also, the second diffraction region 252 is configured so as to
function as described above as to the optical beams of the first and
second wavelengths, and is configured such that the diffracted light of
an order that does not condense the optical beam of the third wavelength
that is transmitted therethrough upon the signal recording face of the
third optical disc via the object lens 234 is dominant by taking into
consideration the influence of flaring and so forth, so even if the
optical beam of the third wavelength that has transmitted the second
diffraction region 252 is input to the object lens 234, this seldom
affects the signal recording face of the third optical disc, i.e., the
second diffraction region 252 can serve so as to markedly reduce the
light quantity of the optical beam of the third wavelength transmitted
through the second diffraction region 252, and condensed on the signal
recording face by the object lens 234 to around zero, and subject the
optical beam of the third wavelength to aperture restriction.

[0413]Incidentally, the above-mentioned first diffraction region 251 is
formed with a size wherein the optical beam of the third wavelength
transmitted through the region thereof is input to the object lens 234 in
the same state as that of the optical beam subjected to aperture
restriction at around NA=0.45, and also, the second diffraction region
252 formed on the outer side of the first diffraction region 251 does not
condense the optical beam of the third wavelength transmitted through
this region on the third optical disc via the object lens 234, so
consequently, the diffraction unit 250 including the first and second
diffraction regions 251 and 252 thus configured serves so as to perform
aperture restriction at around NA=0.45 as to the optical beam of the
third wavelength. An arrangement has been made here wherein with the
diffraction unit 250, aperture restriction of numerical aperture NA of
around 0.45 is performed as to the optical beam of the third wavelength,
but the numerical aperture restricted by the above arrangement is not
restricted to this.

[0414]Specifically, the second diffraction region 252 has, as shown in
FIG. 39 and FIG. 40B, a ring zone shape centered on the optical axis,
which is formed such that the cross-sectional shape of this ring zone
becomes a blazed shape of a predetermined depth (hereafter, also referred
to as "groove depth") d as to the reference face.

[0415]Also, description will be made here assuming that the second
diffraction region having a diffraction structure is formed such that the
cross-sectional shape of the ring zone is a blazed shape, but as long as
this diffraction structure is configured such that the optical beam of a
predetermined order is dominant as to the optical beam of each wavelength
as described above, for example, a diffraction region 252B may be formed,
as shown in FIG. 47, which has a ring zone shape centered on the optical
axis, and the cross-sectional shape of this ring zone is configured as to
the reference face such that staircase forms having a predetermined depth
d, and a predetermined number of steps S are formed consecutively in the
radial direction.

[0416]As shown in FIG. 47, the diffraction region 252B in the case of a
staircase form being formed as a middle ring zone has a ring zone shape
centered on the optical axis, and the cross-sectional shape of this ring
zone is configured wherein staircase forms having a predetermine depth d
and a predetermined number of steps S are consecutively formed in the
radial direction. Note that the second diffraction region 252B has
different numeric values of d and/or S as compared with those in the case
of the first diffraction region 251, i.e., the second diffraction
structure different from the first diffraction structure provided in the
first diffraction region 251 is formed. For example, the diffraction
structure of the second diffraction region 252B shown in FIG. 47 is a
diffraction structure wherein the number of steps S is set to 5 (S=5),
staircase forms including first through fifth step portions 252Bs1,
252Bs2, 252Bs3, 252Bs4, and 252Bs5 each having generally the same depth
(d/3) are consecutively formed in the radial direction, and first through
sixth diffractive faces 252Bf1, 252Bf2, 252Bf3, 252Bf4, 252Bf5, and
252Bf6 are formed generally with the same interval (d/5) in the optical
axis direction.

[0417]Also, in a case wherein the second diffraction region 252 diffracts
the optical beam of the first wavelength which is transmitted
therethrough such that diffracted light of the k1m'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, diffracts
the optical beam of the second wavelength which is transmitted
therethrough such that diffracted light of the k2m'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, and
diffracts the optical beam of the third wavelength which is transmitted
therethrough such that diffracted light of the k3m'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, the
diffraction orders k1m, k2m, and k3m are set so as to satisfy relations
determined from the following first through third perspectives.

[0418]First, the first perspective will be described. As the first
perspective, the diffraction orders k1m, k2m, and k3m which become the
maximum diffraction efficiency do not satisfy the relational expression
of the above-mentioned Expression (1) (let us say that x of k1x, k2x, k3x
within this conditional expression with a middle ring zone is x=m). This
is because with a middle ring zone region, in the case of k1m, k2m, and
k3m satisfying Expression (1), the diffracted light of the order k3m of
the third wavelength is formed on the signal recording face of the third
optical disc. In such a case, the aperture restriction as to the third
wavelength cannot be realized.

[0419]In other words, an arrangement may be made wherein the second
diffraction region 252 generates the diffraction efficiency of the
diffracted light of the diffraction orders k1m and k2m of the optical
beams of the first and second wavelengths in a high state via the object
lens 234 so as to condense light to form a suitable spot on the signal
recording faces of the first and second optical discs, and suppresses the
diffraction efficiency of the diffraction order of the optical beam of
the third wavelength condensed on the signal recording face of the third
optical disc as much as possible so as to have an aperture restriction
function, but the relation of Expression (1) is not satisfied here,
thereby shifting the optical beam of the diffraction order according to
the optical beam of this third wavelength from a state where a focal
point is imaged on the signal recording face of the third optical disc to
further reduce the light quantity of the optical beam condensed on the
signal recording face of the third optical disc substantially. Hereafter,
a position where the optical beam of a predetermined wavelength is formed
via the object lens 234 is shifted from the signal recording face of the
corresponding optical disc, thereby reducing the light quantity of the
optical beam of this wavelength condensed on the signal recording face
substantially, which will be called flaring, and the details thereof will
be described later.

[0420]Note that with regard to the third wavelength, there is a need to
make an arrangement such that with not only the diffraction order k3m
having the maximum diffraction efficiency but also all of the diffraction
orders having predetermined diffraction efficiency, the diffraction
orders thereof will be replaced with k3m, and the above-mentioned
relational expression is set so as not to be satisfied along with k1m,
and k2m. This is because if the diffracted light of the diffraction order
having predetermined efficiency satisfies the relation of Expression (1),
the diffracted light thereof is condensed by the object lens, and
accordingly aperture restriction cannot be performed appropriately. Now,
let us say that the term "predetermined diffraction efficiency" means an
efficiency level wherein the optical beam transmitted through this region
is irradiated on the optical disc, the optical beam reflected at the
optical disc is input to the photoreceptor portion, and this becomes
noise when the return light of the optical beam transmitted within a
regular aperture range is detected at the photoreceptor unit, and in
other words, means an efficiency level wherein aperture restriction
cannot be performed appropriately.

[0421]On the other hand thereof, like this first perspective, the
diffraction orders k1m, k2m, and k3m that do not satisfy the relational
expression of Expression (1) are selected, whereby aperture restriction
as to the third wavelength can be performed appropriately.

[0422]Next, the second perspective will be described. As the second
perspective, in a case wherein similar to the description regarding inner
ring zones, the selected order is too great, the steps, groove width, and
blazed depth of the diffraction structure becomes deeper. When the depth
of the diffraction structure becomes deeper, there is a possibility that
formation precision deteriorates, and also there is problem wherein the
optical path length enhancement increase effect according to change in
temperature increases, temperature diffraction efficiency properties
deteriorate. It is desirable and common from such reasons to select an
diffraction order up to around 3rd to 4th order.

[0423]Next, the third perspective will be described. As the third
perspective, similar to the description regarding inner ring zones, when
diffraction efficiency calculation as described later is performed, there
is a need to satisfy that the depth d is equal to or smaller than a
suitable size, and formation can be made with this depth size. Further,
the depth d needs to be equal to or smaller than at least 15 μm.
Predetermined diffraction orders k1m and k2m need to be selected so as to
satisfy the above-mentioned first through third perspectives at the
second diffraction region, and for example, a combination of (k1m,
k2m)=(+1, +1), (-1, -1), (0, +2), (0, -2), (0, +1), (0, -1), (+1, 0), and
(-1, 0) (hereafter, this combination is referred to as "diffraction order
combination A of middle ring zones"), and a combination of (k1m,
k2m)=(+3, +2), (-3, -2), (+2, +1), and (-2, -1) (hereafter, this
combination is referred to as "diffraction order combination B of middle
ring zones") are optimal arrangement examples. Now, the following Table 7
shows the above-mentioned functions of middle ring zones, staircase forms
when taking into consideration diffraction efficiency and so forth,
diffraction structure form selected from blazed forms, number of steps S
("∞" in the case of a blazed form), and groove depth d, when
selecting the diffraction order combination A or B of middle ring zones.
As shown in Table 7, with the diffraction order combination A of middle
ring zones, there is groove depth whereby the optimal diffraction
efficiency can be obtained with the diffraction structure of the
staircase form which is a so-called step form, i.e., it can be said that
this combination is a combination suitable for the diffraction structure
of the staircase form. In Table 7, MA1 through MA4 show respective
combinations of the combination A, and MB1 through MB2 show respective
combinations of the combination B. Note that in the case of the
combination A, the optimal solution can be obtained even with an
non-cyclical structure. Also, with the diffraction order combination B of
middle ring zones, there is groove depth whereby the optimal diffraction
efficiency can be obtained with the diffraction structure of the blazed
form, i.e., it can be said that this combination is a combination
suitable for the diffraction structure of the blazed form. Note that in
Table 7, with the diffraction structure suitable for the above-mentioned
combination of the diffraction orders k1m and k2m, a diffraction order
k3m of which the diffraction efficiency of the optical beam of the third
wavelength becomes the maximum efficiency, and a diffraction order having
the second largest diffraction efficiency as so-called unwanted light is
shown as "k3m'". Also, in Table 7, diffraction efficiency eff1, eff2, and
eff3 of the orders k1m, k2m, and k3m of the respective wavelengths, and
also diffraction efficiency eff3' of the diffraction order k3m' of the
third wavelength are shown. Further, in a case wherein with each example,
the deviation amount Δ from the spherical aberration correction
line of the plot Pλ3 of the third wavelength, and also in the case
of plotting the diffraction order k3m' of the third wavelength similarly,
the deviation amount from the spherical aberration correction line of
this plot point is shown as "Δ'". Note that the combinations of
Table 7 and the orders k1m, k2m, k3m, and k3m' within later-described
Table 8 are combinations of decoding in the same order. Also, in Table 7,
the asterisk "*" indicates that with eff3', diffraction efficiency is
low, which effects no problem.

[0424]As shown in Table 7, with the above-mentioned combinations A and B,
in either case, diffraction efficiency is sufficiently ensured, and also
in the case of the diffraction efficiency of the third wavelength
existing, the deviation amount Δ is sufficiently great, i.e.,
spherical aberration is provided greatly to the optical beam of the third
wavelength, which does not contribute to image formation, thereby
confirming that the aperture restriction function is exhibited. This
means that flaring effects are obtained. Note that in Table 7, with the
combinations A and B, it goes without saying that there is a combination
including multiple solutions as to the groove depth d and number of steps
S, but an example of the groove depth d and number of steps S thereof is
shown as a typical example thereof.

[0425]Also, the diffraction orders k1m and k2m to be selected at the
second diffraction region 252 that satisfy the above-mentioned first
through third perspectives are not restricted to the above combinations,
and for example, a combination of (k1m, k2m)=(+1, -1) and (-1, +1)
(hereafter, this combination is referred to as "diffraction order
combination C of middle ring zones"), and a combination of (k1m,
k2m)=(+1, +1) and (-1, -1) (hereafter, this combination is referred to as
"diffraction order combination D of middle ring zones") are also optimal
arrangement examples. Now, when selecting the diffraction order
combination C or D of the middle ring zones, the above-mentioned
functions of the middle ring zones, staircase form to be selected when
taking into consideration diffraction efficiency and so forth, the form
of the diffraction structure to be selected from blazed forms, number of
steps S, and groove width d are shown in MC1 and MD1 in the following
Table 8. Now, as shown in Table 8, with the diffraction order combination
C of the middle ring zones, there is groove depth whereby the optimal
diffraction efficiency can be obtained with the diffraction structure of
the staircase form which is a so-called step form, i.e., it can be said
that this combination is a combination suitable for the diffraction
structure of the staircase form. Also, with the diffraction order
combination D of middle ring zones, there is groove depth whereby the
optimal diffraction efficiency can be obtained with the diffraction
structure of the blazed form, i.e., it can be said that this combination
is a combination suitable for the diffraction structure of the blazed
form. Note that "k1m", "k2m", "k3m", "k3m", "eff1", "eff2", "eff3",
"eff3'", "d", "S", "Δ", and "Δ'" shown in Table 8 are the
same as those described above with reference to Table 7.

[0426]As shown in Table 8, with the above-mentioned combinations C and D,
in either case, diffraction efficiency is sufficiently ensured. Note that
with the example shown in Table 8, the deviation amount Δ or
Δ' is not sufficiently great amount as compared to the example
shown in Table 7, but comparatively low diffraction efficiency eff3 and
eff3', and a certain level of separation amount Δ and Δ' are
obtained, so influence of unwanted light can sufficiently be reduced
while realizing aperture restriction, for example, using a method for
setting return magnification of an optical system greatly, or the like.

[0427]As described above, with the second diffraction region 252 serving
as an inner ring zone, from the above-mentioned first through fourth
perspectives, the diffraction order combination A, B, C, or D of inner
ring zones such as describe above can be selected, and such a diffraction
order is selected, whereby the optical beams of the first and second
wavelengths can be condensed on the signal recording face of the
corresponding optical disc with high diffraction efficiency in a state in
which spherical aberration is reduced, and also the diffracted light of
the high diffraction order of diffraction efficiency is prevented from
being condensed on the signal recording face of the third optical disc as
to the optical beam of the third wavelength, thereby enabling aperture
restriction to be performed.

[0428]Note that, as described above, with a middle ring zone, the second
diffraction region 252B of the staircase form may be employed instead of
the second diffraction region 252 of the blazed form. This is because, as
described in the above description of inner ring zones, while the
staircase form (step structure) is advantageous to reduce influence of
unwanted light, middle ring zones are provided outer side than inner ring
zones, and the lens curved face is steep, so the blazed form (blazed
structure) is advantageous from the perspective of manufacturing. That is
to say, with a middle ring zone, an advantageous structure needs to be
selected while taking into consideration the relation with other
structures with subtle balance between influence of unwanted light and
advantages from the perspective of manufacturing.

[0429]Now, description will be made regarding flaring with the second
diffraction region 252, and the structure thereof. With the above
description of the first diffraction region 251, description has been
made wherein it is required to satisfy the above-mentioned conditional
expression
(λ1×k1x-λ2×k2x)/(t1-t2)≈(λ1×k-
1x-λ3×k3x)/(t1-t3), but this conditional expression (with a
middle ring zone, let us say that x of k1x, k2x, and k3x within this
conditional expression is x=m) is also taken into consideration with the
second diffraction region 252. With the second diffraction region 252
serving as a middle ring zone, when taking into consideration a function
for generating the diffraction light of the diffraction orders k1m and
k2m of the optical beams of the first and second wavelengths to be
condensed via the object lens 234 in a state wherein diffraction
efficiency is high so as to form a suitable spot on the signal recording
faces of the first and second optical discs such as described above,
Pλ1 and Pλ2 to be plotted need to be positioned on a design
line, but further, in order to perform flaring regarding the third
wavelength, there is a need to select a design line so as to make
Pλ3 deviate from this design line intentionally. That is to say,
the object lens 234 is configured based on the design line that deviates
from the design line regarding Pλ3, whereby the diffracted light of
the relevant diffraction order of the optical beam of the third
wavelength can be shifted from a state wherein a focal point is imaged on
the signal recording face of the third optical disc, the light quantity
of the optical beam of the third wavelength condensed on the signal
recording face of the third optical disc can be reduced substantially,
whereby aperture restriction as to the optical beam of the third
wavelength as described above can be performed in a sure and excellent
manner. Specifically, Pλ3 deviates from the design line L22 in the
case of (k1m, k2m, k3m)=(+3, +2, +2) such as shown in FIG. 48, and in
addition to the effects wherein the diffraction efficiency of the
diffracted light of the relevant order of the third wavelength can be
reduced according to the diffraction structure formed in the second
diffraction region 252 expected from the beginning, the flaring effects
are further obtained, and according to such a configuration, the light
quantity of the optical beam of the third wavelength can be further
prevented from being input to the third optical disc.

[0430]With the third diffraction region 253 which is an outer ring zone,
the third diffraction structure is formed, which has a ring zone shape,
predetermined depth, and a structure different from the first and second
diffraction structures, and the third diffraction region 253 diffracts
the optical beam of the first wavelength that is transmitted therethrough
such that diffracted light of an order which forms an appropriate spot on
the signal recording face of the first optical disc via the object lens
234 is dominant, i.e., such that maximum diffraction efficiency is
manifested regarding diffracted light of other orders.

[0431]Also, the third diffraction region 253 diffracts the optical beam of
the second wavelength that is transmitted therethrough such that
diffracted light of an order other than the order which condenses light
so as to form an appropriate spot on the signal recording face of the
second optical disc via the object lens 234 is dominant, i.e., such that
maximum diffraction efficiency is manifested regarding diffracted light
of other orders, by way of the third diffraction structure. If it puts in
another way regarding this point, in light of later-described flaring
operation and so forth, the third diffraction region 253 diffracts the
optical beam of the second wavelength that is transmitted therethrough
such that diffracted light of an order which forms no appropriate spot on
the signal recording face of the second optical disc via the object lens
234 is dominant, by way of the third diffraction structure. Note that the
third diffraction region 253 can sufficiently reduce the diffraction
efficiency of diffracted light of an order which forms an appropriate
spot condensed on the signal recording face of the second optical disc
via the object lens 234 for the optical beam of the second wavelength
that is transmitted therethrough, by way of the third diffraction
structure.

[0432]Also, the third diffraction region 253 diffracts the optical beam of
the third wavelength that is transmitted therethrough such that
diffracted light of an order other than which forms an appropriate spot
condensed on the signal recording face of the third optical disc via the
object lens 234 is dominant, i.e., such that maximum diffraction
efficiency is manifested regarding diffracted light of other orders, by
way of the third diffraction structure. If it puts in another way
regarding this point, in light of later-described flaring operation and
so forth, the third diffraction region 253 diffracts the optical beam of
the third wavelength that is transmitted therethrough such that
diffracted light of an order which forms no appropriate spot on the
signal recording face of the third optical disc via the object lens 234
is dominant, by way of the third diffraction structure. Note that the
third diffraction region 253 can sufficiently reduce the diffraction
efficiency of diffracted light of an order which forms an appropriate
spot condensed on the signal recording face of the third optical disc via
the object lens 234 for the optical beam of the third wavelength that is
transmitted therethrough, by way of the third diffraction structure.

[0433]Thus, with the third diffraction region 253, there is formed a
diffraction structure suitable for the diffracted light of a
predetermined order being dominant as to the optical beam of the
above-mentioned respective wavelengths, thereby enabling spherical
aberration to be corrected and reduced at the time of the optical beams
of the first wavelength serving as the diffracted light of a
predetermined order that is transmitted therethrough being condensed on
the signal recording face of the optical disc via the object lens 234.

[0434]Also, the third diffraction region 253 serves as described above as
to the optical beam of the first wavelength, and is configured such that
the diffracted light of an order that does not condense the optical beams
of the second and third wavelengths that is transmitted therethrough upon
the signal recording faces of the second and third optical discs via the
object lens 234 is dominant by taking into consideration the influence of
flaring and so forth, so even if the optical beams of the second and
third wavelengths that have transmitted the third diffraction region 253
is input to the object lens 234, this seldom affects the signal recording
faces of the second and third optical discs, i.e., the third diffraction
region 253 can serve so as to markedly reduce the light quantity of the
optical beams of the second and third wavelengths transmitted through the
third diffraction region 253, and condensed on the signal recording face
by the object lens 234 to around zero, and subject the optical beam of
the second wavelength to aperture restriction. Note that the third
diffraction region 253 can serve so as to subject the optical beam of the
third wavelength to aperture restriction together with the
above-mentioned second diffraction region 252.

[0435]Incidentally, the above-mentioned second diffraction region 252 is
formed with a size wherein the optical beam of the second wavelength
transmitted through the region thereof is input to the object lens 234 in
the same state as that of the optical beam subjected to aperture
restriction at around NA=0.6, and also, the third diffraction region 253
formed on the outer side of the second diffraction region 252 does not
condense the optical beam of the second wavelength transmitted through
this region on the optical disc via the object lens 234, so consequently,
the diffraction unit 250 including the second and third diffraction
regions 252 and 253 thus configured serves so as to perform aperture
restriction at around NA=0.6 as to the optical beam of the second
wavelength. An arrangement has been made here wherein with the
diffraction unit 250, aperture restriction of numerical aperture NA of
around 0.6 is performed as to the optical beam of the second wavelength,
but the numerical aperture restricted by the above arrangement is not
restricted to this.

[0436]Also, the third diffraction region 253 is formed of a size such that
the optical beam of the first wavelength which has been transmitted
through the region thereof is input to the object lens 234 in the same
state as an optical beam which has been subjected to aperture restriction
at around NA=0.85, and since there is no diffraction structure formed on
the outer side of this third diffraction region 253, this does not allow
condensation of the optical beam of the first wavelength which has been
transmitted through this region on the first optical disc, and the
diffraction unit 250 which has the third diffraction region 253
configured thus functions so as to restrict the numerical aperture of the
optical beam of the first wavelength to around NA=0.85. Note that with
the first wavelength optical beam transmitted through the third
diffraction region 253, light of 1st and 4th diffraction orders is
dominant, so the zero-order light transmitted through the region outside
the third diffraction region 253 almost never passes through the object
lens 234 to be condensed on the first optical disc, but in cases wherein
this zero-order does pass through the object lens 234 and is condensed on
the first optical disc, a configuration may be provided to perform
aperture restriction by providing, at the region outside of the third
diffraction region 253, either a shielding portion for shielding optical
beams passing through, or a diffraction region having a diffraction
structure wherein optical beams of orders other than the order of the
optical beam passing through the object lens 234 to be condensed on the
first optical disc are dominant. It should be noted however, that while
in this arrangement of the diffraction unit 250, the optical beam of the
first wavelength is subjected to aperture restriction around NA=0.85, but
the present invention is not restricted to this, i.e., numerical aperture
restriction due to the above configuration is not limited to this.

[0437]Specifically, the third diffraction region 253 has, as shown in FIG.
39 and FIG. 40C, a ring zone shape centered on the optical axis, which is
formed such that the cross-sectional shape of this ring zone becomes a
blazed shape of a predetermined depth d as to the reference face.

[0438]With the third diffraction region 253 which is an outer ring zone, a
blazed structure is employed, as described above. This is because with
the outer ring zone provided on the outermost side, the lens curved face
has the most steep curvature, and providing a structure other than a
blazed structure is disadvantageous from the perspective of
manufacturing. Also, there is no need to take into consideration problems
such as unwanted light, efficiency, and so forth as described above, so
sufficient performance can be obtained with a blazed structure.
Description will be made below regarding the respective orders to be
selected.

[0439]Also, in a case wherein the third diffraction region 253 diffracts
the optical beam of the first wavelength which is transmitted
therethrough such that diffracted light of the k1o'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, diffracts
the optical beam of the second wavelength which is transmitted
therethrough such that diffracted light of the k2o'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, and
diffracts the optical beam of the third wavelength which is transmitted
therethrough such that diffracted light of the k3o'th order is dominant,
i.e., such that the diffraction efficiency thereof is maximum, when
selecting the diffraction orders k1o, k2o, and k3o, only the order of the
first wavelength and diffraction efficiency need to be taken into
consideration.

[0440]This is because the condensed points of the second and third
wavelengths having predetermined diffraction efficiency are subjected to
flaring so as to be shifted from the state wherein an image is formed,
whereby the light quantity of the optical beams condensed on the signal
recording face of the second and third optical discs can be reduced
substantially, and accordingly, flexibility is high, and conditions are
alleviated.

[0441]From the perspectives described above, with the third diffraction
region 253, predetermined diffraction orders k1o, k2o, and k3o need to be
selected, as for an example thereof, like a later-described first
embodiment, in the case of (k1o, k2o, k3o)=(+4, +2, +2), the
above-mentioned respective perspectives are satisfied, the corresponding
efficiency can be obtained.

[0442]Now, description will be made regarding flaring with the third
diffraction region 253, and the structure thereof. With the above
description of the first diffraction region 251, description has been
made wherein it is required to satisfy the conditional expression
(λ1×k1x-λ2×k2x)/(t1-t2)≈(λ1×k-
1x-λ3×k3x)/(t1-t3), but this conditional expression (with an
outer ring zone, let us say that x of k1x, k2x, and k3x within this
conditional expression is x=o) is also taken into consideration with the
third diffraction region 253. With the third diffraction region 253
serving as an outer ring zone, when taking into consideration a function
for generating the diffraction light of the diffraction order ko of the
optical beams of the first wavelength to be condensed via the object lens
234 in a state wherein diffraction efficiency is high so as to form a
suitable spot on the signal recording face of the first optical disc such
as described above, Pλ1 to be plotted needs to be positioned on a
design line, but further, in order to subject the second or third
wavelength, or the second and third wavelengths to flaring, there is a
need to select a design line so as to make the corresponding Pλ2
and Pλ3 deviate from this design line intentionally.

[0443]That is to say, the object lens 234 is configured based on the
design line that deviates from the design line regarding Pλ2,
whereby the diffracted light of the relevant diffraction order of the
optical beam of the second wavelength can be shifted from a state wherein
a focal point is imaged on the signal recording face of the second
optical disc, the light quantity of the optical beam of the second
wavelength condensed on the signal recording face of the second optical
disc can be reduced substantially, whereby aperture restriction as to the
optical beam of the second wavelength as described above can be performed
in a sure and excellent manner. Also, the object lens 234 is configured
based on the design line that deviates from the design line regarding
Pλ3, whereby the diffracted light of the relevant diffraction order
of the optical beam of the third wavelength can be shifted from a state
wherein a focal point is imaged on the signal recording face of the third
optical disc, the light quantity of the optical beam of the third
wavelength condensed on the signal recording face of the third optical
disc can be reduced substantially, whereby aperture restriction as to the
optical beam of the third wavelength as described above can be performed
in a sure and excellent manner. Also, the object lens 234 is configured
based on the design line that deviates from the design line regarding
Pλ2 and Pλ3, whereby both effects described above, i.e., the
light quantity of the optical beams of the second and third wavelengths
condensed on the signal recording face of the corresponding optical disc
can be reduced.

[0444]Specifically, both of Pλ2 and Pλ3 deviate from the
design line L23 in the case of (k1o, k2o, k3o)=(+4, +2, +2) such as shown
in FIG. 49, and in addition to effects wherein the diffraction efficiency
of the diffracted light of the orders of the second and third wavelengths
can be reduced according to the diffraction structure formed in the third
diffraction region 253 expected from the beginning, the flaring effects
are further obtained, and according to such a configuration, the light
quantity of the optical beams of the second and third wavelengths can be
further prevented from being input to the second and third optical discs.

[0445]As a specific embodiment of the diffraction unit 250 including the
first diffraction region 251 which is an inner ring zone, the second
diffraction region 252 which is a middle ring zone, and the third
diffraction region 253 which is an outer ring zone, the diffraction order
of the diffracted light of an order that is dominant as to the optical
beam of each wavelength, and the diffraction efficiency of the diffracted
light of the diffraction order thereof will be shown in Table 9 and
later-described Table 10 by listing specific numeric values regarding the
depth d and number of steps S according to the blazed or staircase form.
Note that Table 9 shows the first embodiment of the diffraction unit 250,
Table 10 shows the second embodiment of the diffraction unit 250, and in
Tables 9 and 10, k1 represents the diffraction orders (k1i, k1m, k1o)
wherein the diffraction efficiency of the optical beam of the first
wavelength at each ring zone becomes the maximum efficiency, i.e., the
diffraction orders wherein condensation is made so as to form a spot
appropriately on the signal recording face of the first optical disc via
the object lens 234, eff1 represents the diffraction efficiency of the
relevant diffraction orders (k1i, k1m, k1o) of the optical beam of the
first wavelength, k2 represents the diffraction orders (k2i, k2m, k2o)
wherein the diffraction efficiency of the optical beam of the second
wavelength becomes the maximum efficiency, and particularly with the
inner and middle ring zones, represents the diffraction orders wherein
condensation is made so as to form a spot appropriately on the signal
recording face of the second optical disc via the object lens 234, eff2
represents the diffraction efficiency of the relevant diffraction orders
(k2i, k2m, k2o) of the optical beam of the second wavelength, k3
represents the diffraction orders (k3i, k3m, k3o) wherein the diffraction
efficiency of the optical beam of the third wavelength becomes the
maximum efficiency, and particularly with the inner ring zone, represents
the diffraction orders wherein condensation is made so as to form a spot
appropriately on the signal recording face of the third optical disc via
the object lens 234, eff3 represents the diffraction efficiency of the
relevant diffraction orders (k3i, k3m, k3o) of the optical beam of the
third wavelength, d represents the groove depth of each diffraction
region, S represents the number of steps in the case of the staircase
form, or "∞" in the case of the blazed form. Also, "*" in Tables 9
and 10 represents a state wherein according to the above-mentioned
flaring, efficiency does not effect a problem.

[0446]Now, the first embodiment shown in Table 9 will be described. With
the inner ring zone according to the first embodiment, as shown in Table
9, when employing a staircase form with the number of steps S=4 and
groove depth d=3.8 (μm), with the diffraction order k1i=+1 of the
optical beam of the first wavelength, the diffraction efficiency is
eff1=0.81, with the diffraction order k2i=-1 of the optical beam of the
second wavelength, the diffraction efficiency is eff2=0.62, and with the
diffraction order k3i=-2 of the optical beam of the third wavelength, the
diffraction efficiency is eff3=0.57. Further specific description of the
inner ring zone according to the first embodiment has been made with
reference to FIGS. 44A through 44C, so detailed description thereof will
be omitted.

[0447]Also, with the middle ring zone according to the first embodiment,
as shown in Table 9, when employing a blazed form (S=∞) with groove
depth d=2.4 (μm), with the diffraction order k1m=+3 of the optical
beam of the first wavelength, the diffraction efficiency is eff1=0.96,
and with the diffraction order k2m=+2 of the optical beam of the second
wavelength, the diffraction efficiency is eff2=0.93. Also, the
diffraction efficiency eff3 of the diffraction order k3m=+2 serving as
the maximum diffraction efficiency of the optical beam of the third
wavelength transmitting this region is around 0.4, but this does not
contribute to image formation since the spot is subjected to flaring as
described above with reference to FIG. 48.

[0448]Next, description will be made further specifically regarding the
middle ring zone according to the first embodiment with reference to
FIGS. 50A through 50C. FIG. 50A is a diagram illustrating change in the
diffraction efficiency of +3rd order diffracted light of the optical beam
of the first wavelength in the case of changing the groove depth d of the
blazed form of the number of steps S=∞, FIG. 50B is a diagram
illustrating change in the diffraction efficiency of +2nd order
diffracted light of the optical beam of the second wavelength in the case
of changing the groove depth d of the blazed form of the number of steps
S=∞, and FIG. 50C is a diagram illustrating change in the
diffraction efficiency of +2nd order diffracted light of the optical beam
of the third wavelength in the case of changing the groove depth d of the
blazed form of the number of steps S=∞. In FIGS. 50A through 50C,
the horizontal axis represents groove depth (nm), and the vertical axis
represents diffraction efficiency (light intensity). At a position where
the horizontal axis is 2400 nm, as shown in FIG. 50A, eff1 is 0.96, and
as shown in FIG. 50B, eff2 is 0.93, and as shown in FIG. 50C, eff3 is
around 0.4, but the spot is subjected to flaring.

[0449]Also, with the middle ring zone in the first embodiment described
above, of the design line in the relation between the above-described
(wavelength×order) and the thickness of the protective layer, the
y-intercept position and inclination with the vertical axis representing
the thickness of the protective layer as the Y axis exhibits flaring
regarding the third wavelength by change due to design of the object
lens. Accordingly, performing appropriate object lens design based on
such a design line enables the quantity of light of the optical beam of
the third wavelength to be further suppressed and excellent aperture
restriction to be performed regarding the optical beam of the third
wavelength. Specifically, as shown in FIG. 48, the middle ring zone in
the first embodiment has the design line indicated by L22 set by plotting
the points Pλ1, Pλ2, and Pλ3 at the diffraction orders
(k1m, k2m, k3m)=(+3, +2, +2). In FIG. 48 the design point Pλ1 of
the first wavelength and the design point Pλ2 of the second
wavelength are positioned on the design line L22, so the aberration of
diffraction light of the diffraction orders k1m and k2m is approximately
zero. On the other hand, the plotted point Pλ3 of the third
wavelength is significantly deviated from the aberration zero design
point, indicating the above-described flaring. Note that in FIG. 48, only
k3m=+2 is shown plotted, but there is deviation from the design line L22
in the same way for other orders in the third wavelength as well.
Consequently, there is uncorrected aberration in the third wavelength,
and consequently, the light quantity of the optical beam of the third
wavelength which has passed through the middle ring zone, that is not
imaged at the signal recording face but input to the third optical disc
can be suppressed. As a result, regardless of the diffraction efficiency
of the optical beam of the third wavelength as shown in FIG. 50, these
optical beams do not contribute to image formation, and accordingly, a
suitable aperture restriction (NA=0.45) can be realized.

[0450]Also, with the outer ring zone according to the first embodiment, as
shown in Table 9, when employing a blazed form (S=∞) with groove
depth d=3.1 (μm), with the diffraction order k1o=+4 of the optical
beam of the first wavelength, the diffraction efficiency is eff1=1.0.
Also, the diffraction efficiency eff2 of the diffraction order k2o=+2
serving as the maximum diffraction efficiency of the optical beam of the
second wavelength transmitting this region is around 0.6, but this does
not contribute to image formation since the spot is subjected to flaring
as described above with reference to FIG. 49. Further, the diffraction
efficiency eff3 of the diffraction order k3o=+2 serving as the maximum
diffraction efficiency of the optical beam of the third wavelength
transmitting this region is around 1.0, but this does not contribute to
image formation since the spot is subjected to flaring as described above
with reference to FIG. 49.

[0451]Next, description will be made further specifically regarding the
outer ring zone according to the first embodiment with reference to FIGS.
51A through 51C. FIG. 51A is a diagram illustrating change in the
diffraction efficiency of +4th order diffracted light of the optical beam
of the first wavelength in the case of changing the groove depth d of the
blazed form of the number of steps S=∞, FIG. 51B is a diagram
illustrating change in the diffraction efficiency of +2nd order
diffracted light of the optical beam of the second wavelength in the case
of changing the groove depth d of the blazed form of the number of steps
S=∞, and FIG. 51C is a diagram illustrating change in the
diffraction efficiency of +2nd order diffracted light of the optical beam
of the third wavelength in the case of changing the groove depth d of the
blazed form of the number of steps S=∞. In FIGS. 51A through 51C,
the horizontal axis represents groove depth (nm), and the vertical axis
represents diffraction efficiency (light intensity). At a position where
the horizontal axis is 3100 nm, as shown in FIG. 51A, eff1 is 1.0, and as
shown in FIG. 51B, eff2 is around 0.6, and as shown in FIG. 51C, eff3 is
around 1.0, but the spot is subjected to flaring.

[0452]Also, with the outer ring zone in the first embodiment described
above as well, in the same way as the case of the middle ring zone in the
first embodiment described above, an arrangement is made wherein the
design line of the object lens is deviated, and flaring is carried out
regarding the second and third wavelengths to perform excellent aperture
restriction. Specifically, as shown in FIG. 49, the outer ring zone in
the first embodiment has the design line indicated by L23 set by plotting
the points Pλ1, Pλ2, and Pλ3 at the diffraction orders
(k1o, k2o, k3o)=(+4, +2, +2). In FIG. 49 the design point Pλ1 of
the first wavelength is positioned on the design line L23, so the
aberration of diffraction light of the diffraction orders k1o is
approximately zero. On the other hand, the plotted points Pλ2 and
Pλ3 of the second and third wavelengths are significantly deviated
from the aberration zero design point, indicating the above-described
flaring. Note that in FIG. 49, only (k2o, k3o)=(+2, +2) is shown plotted,
but there is deviation from the design line L23 in the same way for other
orders in the second and third wavelengths as well. Consequently, there
is uncorrected aberration in the second wavelength, and consequently, the
light quantity of the optical beams of the second and third wavelengths
which have passed through the outer ring zone, that is not imaged at the
signal recording face but input to the second and third optical discs can
be suppressed. As a result, regardless of the diffraction efficiency of
the optical beam of the second wavelength as shown in FIG. 51, this
optical beam does not contribute to image formation, and accordingly, a
suitable aperture restriction (NA=0.6) can be realized. Also, regardless
of the diffraction efficiency of the optical beam of the third wavelength
as shown in FIG. 51, this optical beam does not contribute to image
formation, and accordingly, a suitable aperture restriction (NA=0.45) can
be realized.

[0453]As described above, with the outer ring zones in the first
embodiment and a later-described second embodiment, the diffraction face
is blazed, so according to this configuration, even in the case of
providing the diffraction grooves to one face of the object lens as
described later, diffraction grooves can be formed relatively easily at
the curved face of the lens face at the perimeter of the lens which has a
steep slope due to being at the outer ring zone.

[0455]Also, with the inner ring zone according to the second embodiment,
as shown in Table 10, when employing a staircase form with the number of
steps S=3 and groove depth d=6.9 (μm), with the diffraction order
k1i=0 of the optical beam of the first wavelength, the diffraction
efficiency is eff1=0.98, and with the diffraction order k2i=-1 of the
optical beam of the second wavelength, the diffraction efficiency is eff2
=0.78, and with the diffraction order k3i=-2 of the optical beam of the
third wavelength, the diffraction efficiency is eff3=0.39.

[0456]Next, description will be made further specifically regarding the
inner ring zone according to the second embodiment with reference to
FIGS. 52A through 52C. FIG. 52A is a diagram illustrating change in the
diffraction efficiency of zero-order diffracted light of the optical beam
of the first wavelength in the case of changing the groove depth d of the
staircase form of the number of steps S=3, FIG. 52B is a diagram
illustrating change in the diffraction efficiency of -1st order
diffracted light of the optical beam of the second wavelength in the case
of changing the groove depth d of the staircase form of the number of
steps S=3, and FIG. 52C is a diagram illustrating change in the
diffraction efficiency of -2nd order diffracted light of the optical beam
of the third wavelength in the case of changing the groove depth d of the
staircase form of the number of steps S=3. In FIGS. 52A through 52C, the
horizontal axis represents groove depth (nm), and the vertical axis
represents diffraction efficiency (light intensity). At a position where
the horizontal axis is 6900 nm, as shown in FIG. 52A, eff1 is 0.98, and
as shown in FIG. 52B, eff2 is 0.78, and as shown in FIG. 52C, eff3 is
0.39.

[0457]Note that with the inner ring zone in the second embodiment as well,
the diffraction orders (k1i, k2i, k3i)=(0, -1, -2) selected here satisfy
the above-mentioned conditional expression (1) (let us say that x of k1x,
k2x, and k3x within the conditional expression is x=i), and are
diffraction orders that can correct and reduce the spherical aberration
on the signal recording face of each optical disc. Further, specifically,
as shown in FIG. 55, the respective plots Pλ1, Pλ2, and
Pλ3 are positioned in a straight line on the straight line L24
serving as a generally design line. Now, strictly, in the same way as
described above with reference to FIG. 42, let us say that the second and
third wavelengths λ2 and λ3 are input as divergent rays,
thereby disposing on a straight line completely.

[0458]With the middle ring zone according to the second embodiment, as
shown in Table 10, when employing a staircase form with the number of
steps S=5 and groove depth d=11.65 (μm), with the diffraction order
k1m=0 of the optical beam of the first wavelength, the diffraction
efficiency is eff1=0.96, and with the diffraction order k2m=-1 of the
optical beam of the second wavelength, the diffraction efficiency is
eff2=0.81. Also, the diffraction efficiency eff3 of the diffraction order
k3m=3 serving as the maximum diffraction efficiency of the optical beam
of the third wavelength transmitting this region is around 0.4, but this
does not contribute to image formation since the spot is subjected to
flaring as described above (see FIG. 56).

[0459]Next, description will be made further specifically regarding the
middle ring zone according to the second embodiment with reference to
FIGS. 53A through 53C. FIG. 53A is a diagram illustrating change in the
diffraction efficiency of zero-order diffracted light of the optical beam
of the first wavelength in the case of changing the groove depth d of the
staircase form of the number of steps S=5, FIG. 53B is a diagram
illustrating change in the diffraction efficiency of -1st order
diffracted light of the optical beam of the second wavelength in the case
of changing the groove depth d of the staircase form of the number of
steps S=5, and FIG. 53C is a diagram illustrating change in the
diffraction efficiency of -3rd order diffracted light of the optical beam
of the third wavelength in the case of changing the groove depth d of the
staircase form of the number of steps S=5. In FIGS. 53A through 53C, the
horizontal axis represents groove depth (nm), and the vertical axis
represents diffraction efficiency (light intensity). At a position where
the horizontal axis is 11650 nm, as shown in FIG. 53A, eff1 is 0.96, and
as shown in FIG. 53B, eff2 is 0.81, and as shown in FIG. 53C, eff3 is
around 0.4, but the spot is subjected to flaring.

[0460]Also, with the middle ring zone in the second embodiment, in the
same way as the case of the middle ring zone in the first embodiment
described above, an arrangement is made wherein the design line of the
object lens is deviated, and flaring is carried out regarding the third
wavelength to perform excellent aperture restriction. Specifically, as
shown in FIG. 56, the middle ring zone in the second embodiment has the
design line indicated by L25 set by plotting the points Pλ1,
Pλ2, and Pλ3 at the diffraction orders (k1m, k2m, k3m)=(0,
-1, -3). In FIG. 56 the design point Pλ1 of the first wavelength
and the design point Pλ2 of the second wavelength are positioned on
the design line L25, so the aberration of diffraction light of the
diffraction orders k1m and k2m is approximately zero. On the other hand,
the plotted point Pλ3 of the third wavelength is significantly
deviated from the aberration zero design point, indicating the
above-described flaring. Note that in FIG. 56, only k3m=-3 is shown
plotted, but there is deviation from the design line L25 in the same way
for other orders in the third wavelength as well. Consequently, there is
uncorrected aberration in the third wavelength, and consequently, the
light quantity of the optical beam of the third wavelength which has
passed through the middle ring zone, that is not imaged at the signal
recording face but input to the third optical disc can be suppressed. As
a result, regardless of the diffraction efficiency of the optical beam of
the third wavelength as shown in FIG. 53, these optical beams do not
contribute to image formation, and accordingly, a suitable aperture
restriction (NA=0.45) can be realized.

[0461]With the outer ring zone according to the second embodiment, as
shown in Table 10, when employing a blazed form (S=∞) with groove
depth d=0.8 (μm), with the diffraction order k1o=+1 of the optical
beam of the first wavelength, the diffraction efficiency is eff1=1.0.
Also, with the diffraction order k2o=+1 serving as the maximum
diffraction efficiency of the optical beam of the second wavelength
transmitting this region, the diffraction efficiency eff2 is around 0.6,
but this does not contribute to image formation since the spot is
subjected to flaring as described above (see FIG. 57). Further, the
diffraction efficiency eff3 of the diffraction order k3o=+1 serving as
the maximum diffraction efficiency of the optical beam of the third
wavelength transmitting this region is around 0.4, but this does not
contribute to image formation since the spot is subjected to flaring as
described above.

[0462]Next, description will be made further specifically regarding the
outer ring zone according to the second embodiment with reference to
FIGS. 54A through 54C. FIG. 54A is a diagram illustrating change in the
diffraction efficiency of +1st order diffracted light of the optical beam
of the first wavelength in the case of changing the groove depth d of the
blazed form of the number of steps S=∞, FIG. 54B is a diagram
illustrating change in the diffraction efficiency of +1st order
diffracted light of the optical beam of the second wavelength in the case
of changing the groove depth d of the blazed form of the number of steps
S=∞ . . . and also illustrating change in diffraction efficiency of
zero order light which is unwanted light, and FIG. 54C is a diagram
illustrating change in the diffraction efficiency of +1st order
diffracted light of the optical beam of the third wavelength in the case
of changing the groove depth d of the blazed form of the number of steps
S=∞ . . . and also illustrating change in diffraction efficiency of
zero order light which is unwanted light. In FIGS. 54A through 54C, the
horizontal axis represents groove depth (nm), and the vertical axis
represents diffraction efficiency (light intensity). At a position where
the horizontal axis is 800 nm, as shown in FIG. 54A, eff1 is 1.0, and as
shown in FIG. 54B, eff2 is around 0.6, but the spot is subjected to
flaring, and as shown in FIG. 54C, eff3 is around 0.4, but the spot is
subjected to flaring.

[0463]Also, with the outer ring zone in the second embodiment described
above, in the same way as the case of the outer ring zone in the first
embodiment described above, an arrangement is made wherein the design
line of the object lens is deviated, and flaring is carried out regarding
the second and third wavelengths to perform excellent aperture
restriction. Specifically, as shown in FIG. 57, the outer ring zone in
the second embodiment has the design line indicated by L26 set by
plotting the points Pλ1, Pλ2, and Pλ3 at the respective
diffraction orders (k1o, k2o, k3o)=(+1, +1, +1). In FIG. 57 the design
point Pλ1 of the first wavelength is positioned on the design line
L26, so the aberration of diffraction light of the diffraction order k1o
is approximately zero. On the other hand, the plotted points Pλ2
and Pλ3 of the second and third wavelengths are significantly
deviated from the aberration zero design point, indicating the
above-described flaring. Note that in FIG. 57, only (k2o, k3o)=(+1, +1)
is shown plotted, but there is deviation from the design line L26 in the
same way for other orders, such as zero order light for example, in the
second and third wavelengths as well. Consequently, there is uncorrected
aberration in the second and third wavelengths, and consequently, the
light quantity of the optical beams of the second and third wavelengths
which have passed through the outer ring zone, that is not imaged at the
signal recording face but input to the second and third optical discs can
be suppressed. As a result, regardless of the diffraction efficiency of
the optical beam of the second wavelength as shown in FIG. 54, this
optical beam does not contribute to image formation, and accordingly, a
suitable aperture restriction (NA=0.6) can be realized. Also, regardless
of the diffraction efficiency of the optical beam of the third wavelength
as shown in FIG. 54, this optical beam does not contribute to image
formation, and accordingly, a suitable aperture restriction (NA=0.45) can
be realized.

[0464]The diffraction units according to the first and second embodiments
having such an inner ring zone, middle ring zone, and outer ring zone,
the relation of the above-mentioned Expression (5B) is satisfied,
diffraction efficiency as to the respective wavelengths is excellent for
all ring zones, whereby sufficient efficiency can be obtained, and it can
be confirmed that the problem of unwanted light is eliminated. Also, as
described above, the inner ring zone is formed in a step form (staircase
form), and the outer ring zone is formed in a blazed form, which is an
advantageous configuration on manufacturing as well.

[0465]Next, the first and second embodiments are confirmed from the
perspective of operating distance and focal distance. Each wavelength of
the first and second embodiments shown in Tables 9 and 10, and the
optical properties as to the corresponding optical disc are shown in the
following Tables 11 and 12. Note that Table 11 corresponds to the first
embodiment, and Table 12 corresponds to the second embodiment. Also,
Tables 11 and 12 show "focal distance", "NA", "effective diameter",
"magnification", and "operating distance" of the object lens, as to the
optical beam of each wavelength and the corresponding optical disc,
"thickness of protective layer" of the optical disc, and "axial
thickness" of the object lens.

[0466]As shown in Tables 11 and 12, with the diffraction units according
to the first and second embodiments, "focal distance" as to the first
wavelength can be suppressed to equal to or smaller than 2.2, and
"operating distance" in the case of employing the optical beam of the
third wavelength can be set to equal to or greater than 0.40, which are
required as described above.

[0467]As described above, with the diffraction units according to the
first and second embodiments, the configurations advantageous to
manufacturing can be provided, the problem of unwanted light can be
eliminated, the conditions for the focal distance and operating distance
of the object lens as to each wavelength can be set desirably, and
predetermined aperture restriction and desired diffraction efficiency can
be obtained as to each wavelength.

[0468]Note that description has been made above assuming that there are
provided the first diffraction region 251 where the diffraction structure
of the staircase form is formed wherein staircase structures including
multiple step portions as inner ring zones are consecutively formed in
the radial direction of the ring zones, the second diffraction regions
252 and 252B where the diffraction structure of the staircase form or
blazed form is formed wherein staircase structures including multiple
step portions as middle ring zones are consecutively formed in the radial
direction of the ring zones, and the third diffraction region 253 where
the diffraction structure of the blazed form is formed as an outer ring
zone, but the present invention is not restricted to this, so the inner
ring zones and middle ring zones may be configured of the diffraction
structure which is an non-cyclical structure as long as this structure
satisfies the above-mentioned relation of a diffraction order to be
selected.

[0469]For example, the first diffraction region may be configured such
that a non-cyclical diffraction structure is formed, wherein an
non-cyclical structure for providing desired phase difference is formed
in the radial direction of the ring zones as described above, and the
second diffraction region may be configured such that a non-cyclical
diffraction structure is formed, wherein an non-cyclical structure for
providing desired phase difference is formed in the radial direction of
the ring zones as described above. In the case of providing a
non-cyclical diffraction structure in the first and second diffraction
regions, flexibility of design is extended, more desirable diffraction
efficiency can be obtained, which is an advantageous structure from the
perspective of the temperature properties of diffraction efficiency.

[0470]Also, as a modification of the above-mentioned first through third
diffraction regions 251, 252, and 253, the third diffraction region may
be formed as a so-called aspherical continuous face. Specifically, an
arrangement may be made wherein predetermined refractive power is applied
to the optical beam of the first wavelength by the refractive power of a
lens curved face instead of such a third diffraction region 253 such as
described above to condense the optical beam on the corresponding optical
disc in a state wherein there is no spherical aberration, and the optical
beams of the second and third wavelengths are subjected to aperture
restriction suitably. In other words, the diffraction unit may be
configured as a diffraction unit including the first diffraction region
251 where the diffraction structure of the staircase form is formed
wherein staircase structures, formed on a region corresponding to the
numerical aperture of the third optical disc, including multiple step
portions as inner ring zones are consecutively formed in the radial
direction of the ring zones, the second diffraction regions 252 and 252B
where the diffraction structure of the staircase form or blazed form is
formed wherein staircase structures, formed on a region corresponding to
the numerical aperture of the second optical disc, including multiple
step portions as middle ring zones are consecutively formed in the radial
direction of the ring zones, and a region formed on a region
corresponding to the numeric aperture of the first optical disc wherein
the optical beam of the first wavelength transmitted therethrough is
condensed on the signal recording face of the corresponding first optical
disc, and the optical beams of the second and third wavelengths
transmitted therethrough are not condensed on the signal recording faces
of the corresponding second and third optical discs.

[0471]With the diffraction unit 250 including the first through third
diffraction regions 251, 252, and 253 thus configured, the optical beams
of the first through third wavelengths transmitted through the first
diffraction region 251 can be diffracted by diffraction power so as to
generate a divergent angle state wherein no spherical aberration occurs
on the signal recording face of the corresponding type of optical disc by
the refractive power of the object lens 234 which is common to the three
wavelengths, a suitable spot can be condensed on the signal recording
face of the corresponding optical disc by the refractive power of the
object lens 234, the optical beams of the first and second wavelengths
transmitted through the second diffraction region 252 can be diffracted
by diffraction power so as to generate a divergent angle state wherein no
spherical aberration occurs on the signal recording face of the
corresponding type of optical disc by the refractive power of the common
object lens, a suitable spot can be condensed on the signal recording
face of the corresponding optical disc by the refractive power of the
object lens 234, the optical beam of the first wavelength transmitted
through the third diffraction region 253 can be diffracted by diffraction
power so as to generate a divergent angle state wherein no spherical
aberration occurs on the signal recording face of the corresponding type
of optical disc by the refractive power of the object lens 234, and a
suitable spot can be condensed on the signal recording face of the
corresponding optical disc by the refractive power of the object lens
234. Here, "a divergent angle state wherein no spherical aberration
occurs" includes a diverged state, converged state, and parallel light
state, and means a state wherein spherical aberration is corrected by the
refractive power of a lens curved face.

[0472]That is to say, with the diffraction unit 250 provided on one face
of the object lens 234 disposed on the optical path between the first
through third emitting units of the optical system the optical pickup 203
and the signal recording face, diffraction power can be applied to
optical beams of respective wavelengths passing through the respective
regions (first through third diffraction regions 251, 252, and 253) so as
to be in a state wherein spherical aberration occurring at the signal
recording face is reduced, so spherical aberration occurring at the
signal recording face when condensing optical beams of the first through
third wavelengths on the signal recording face of the respective
corresponding optical discs using the common object lens 234 in the
optical pickup 203 can be minimized, which is to say that
three-wavelength compatibility of the optical pickup 203 using three
types of wavelengths for three types of optical discs and the common
object lens 234 can be realized, wherein information signals can be
recorded to and/or played from respective optical discs.

[0473]Also, the object lens 234 having the diffraction unit 250 configured
of the first through third diffraction regions 251, 252, and 253 as
described above is configured having the relation k1i≧k2i>k3i
for the diffraction orders (k1i, k2i, k3i) selected by the first
diffraction region 251 serving as the inner ring zone so as to be
dominant and condensed on the signal recording face of the corresponding
optical disc via the object lens 234, so making diffracted light of an
order regarding which spherical aberration can be suitably reduced
dominant, enables a suitable spot to be condensed on the signal recording
face of the optical discs corresponding to the optical beams of each
wavelength, and realize a suitable state for the operating distance for
using the optical beams of each wavelength and a focal distance for each
wavelength, which is to say in the case of using the third wavelength k3
the focal distance can be prevented from becoming too long as to the
first wavelength λ1 in order to ensure operating distance thereof,
thereby preventing problems such as the lens diameter of the object lens
being large, the overall size of the optical pickup being large, and so
forth. Accordingly, the object lens 234 having the diffraction unit 250
realizes condensing optical beams of each wavelength to form a suitable
spot on the signal recording face of the corresponding optical discs with
high light use efficiency without increasing the size of the optical
parts and optical pickup by ensuring a suitable operating distance and
focal distance, which is to say that three-wavelength compatibility of
the optical pickup using three types of wavelengths for three types of
optical discs and the common object lens 234 can be realized, wherein
information signals can be suitably recorded to and/or played from
respective optical discs.

[0474]Also, the object lens 234 having the diffraction unit 250 such as
described above is configured such that, of the diffraction orders
selected by the first diffraction region 251 serving as the inner ring
zone so as to be dominant and condensed on the signal recording face of
the corresponding optical disc via the object lens, k1i and k3i are (-2,
-3), (-1, -2), (-1, -3), (0, -2), (0, -3), (1, -2), (1, -3), (2, -1), (2,
-2), (2, -3), (3, 0), (3, -1), (3, -2), or (3, -3), so making diffracted
light of an order regarding which spherical aberration can be suitably
reduced enables a suitable spot to be condensed on the signal recording
face of the optical discs corresponding to the optical beams of each
wavelength, and realize a suitable state for the operating distance for
using the optical beams of each wavelength and a focal distance for each
wavelength, which is to say in the case of using the third wavelength
λ3 the focal distance can be prevented from becoming too long as to
the first wavelength λ1 in order to ensure operating distance
thereof, thereby preventing problems such as the lens diameter of the
object lens being large, the overall size of the optical pickup being
large, and so forth, and additionally, as described above with regard to
the third perspective for configuring the inner ring zone, the
configuration is advantageous from a manufacturing viewpoint in that the
necessary depth of the grooves is prevented from becoming too deep,
whereby the manufacturing process can be simplified, and also
deterioration of forming precision can be prevented. Accordingly, the
object lens 234 having the diffraction unit 250 can realize condensing
optical beams of each wavelength on the signal recording face of the
corresponding optical discs to form a suitable spot with high light use
efficiency without increasing the size of the optical parts and optical
pickup by ensuring a suitable operating distance and focal distance, and
also simplifies manufacturing process and prevents deterioration of
forming precision.

[0475]Also, the object lens 234 having the diffraction unit 250 such as
described above is configured such that the first diffraction region 251
has formed a staircase form diffraction structure wherein a staircase
structure with multiple steps continue in the radial direction of the
ring zones, and the third diffraction region 253 has a blazed diffraction
structure formed. The object lens 234 having the diffraction unit 250 has
the inner ring zone, which needs to provide the first through third
wavelengths with a diffraction power so as to be in a predetermined
state, and also have high diffraction efficiency, formed in a stepped
shape, thereby suppressing the quantity of diffracted light of unwanted
light, preventing deterioration of jittering and the like due to unwanted
light being received at the photosensor, and also, even in cases of a
certain amount of diffracted light of unwanted light occurring, unwanted
light being received at the time of focusing leading to deterioration of
jittering and the like can be prevented by making the diffraction order
of the unwanted light to be a deviated order with great diffraction angle
difference, that is other than an adjacent diffraction order of the focus
light. Also, the object lens 234 having the diffraction unit 250 has a
configuration has the outer ring zone provided integrally on one face of
the object lens and also provided on the outermost side thereof, formed
in a blazed form, which is an advantageous structure in the case of
forming a diffraction structure at portions having an extremely steep
lens curved surface, such as with a three-wavelength-compatible lens,
whereby manufacturing can be facilitated and deterioration in forming
precision can be prevented.

[0476]Also, the object lens 234 having the diffraction unit 250 such as
described above is configured such that the optical beam of the first
wavelength at the time of input to the incident side of the object lens
234 is an infinite optical system, i.e., generally parallel light, and
the optical beams of the second and third wavelengths are input as a
finite optical system, i.e., as divergent light, whereby, as described
with reference to FIGS. 41, 42, and 55, optical beams passing through the
first diffraction region 251 which is the inner ring zone where there is
need to take into consideration the possibility of spherical aberration
correction can be suitably condensed on the signal recording face of the
optical disc in a state of high diffraction efficiency and no spherical
aberration as predetermined diffraction efficiency as to the selected
diffraction orders k1i, k2i, and k3i for the three wavelengths. Further,
due to the configuration wherein the optical beam of the first wavelength
at the time of input to the incident side of the object lens is generally
parallel light and the optical beams of the second and third wavelengths
are input as divergent light, the object lens 234 having the diffraction
unit 250 has improved freedom for flaring at the middle ring zone and
outer ring zone as described with reference to FIGS. 48, 49, 56, and 57,
and by improving freedom and benefiting from the advantages of flaring,
the freedom of diffraction structure selection of the middle ring zone
and outer ring zone is improved, i.e., higher efficiency can be obtained,
and also the stricture itself is simplified, further enabling
deterioration in formation precision thereof to be prevented. Thus, due
to the configuration wherein the optical beam of the first wavelength at
the time of input to the incident side of the object lens 234 is
generally parallel light and the optical beams of the second and third
wavelengths are input as divergent light, the object lens 234 having the
diffraction unit 250 can realize suitably condensing light of each
wavelength at the signal recording face of the corresponding optical disc
in a state of high diffraction efficiency and no spherical aberration,
with a simpler configuration.

[0477]Note that in the event that the diffraction unit 250 is to be
provided to a diffraction optical element 235B separate from the object
lens as described later (see FIG. 58B), the same advantages can be had
with a configuration wherein, of the object lens and the diffraction
optical element to which the diffraction unit has been provided, the
element positioned at the side closer to the first through third emitting
units is configured such that the optical beam of the first wavelength at
the time of input to the incident side thereof is generally parallel
light and the optical beams of the second and third wavelengths are input
as divergent light.

[0478]Further, the object lens 234 having the diffraction unit 250 such as
described above is configured such that the diffraction orders (k1i, k2i,
k3i) of light selected by the first diffraction region 251 serving as the
inner ring zone and made dominant, and condensed onto the signal
recording face of the corresponding optical disc via the object lens 234,
are (1, -1, -2), (0, -1, -2), (1, -2, -3) or (0, -2, -3), whereby
spherical aberration at each wavelength described with respect to the
first perspective can be reduced when configuring the inner ring zone,
the operating distance and focal distance at each wavelength described
with respect to the second perspective can be made optimal, a
configuration which is advantageous from the aspect of manufacturing as
described with respect to the third and fourth perspectives can be
realized, and further, the diffraction efficiency of the diffraction
orders selected for each wavelength can be set sufficiently high, and
also diffraction efficiency of unwanted light can be suppressed due to
enabling configuration with the stepped form, so adverse effects of
unwanted light can be maximally suppressed since the diffraction
efficiency of the adjacent diffraction order can be suppressed to a low
level. Accordingly, the object lens 234 having the diffraction unit 250
realizes condensing light for a suitable spot on the signal recording
face of corresponding optical discs with high light use efficiency, using
a more advantageous configuration taking into consideration a more
specific configuration and the advantages of reduction in size and of the
configuration.

[0479]Further, with the object lens 234 having the diffraction unit 250
such as described above, when the diffraction orders (k1i, k2i, k3i) of
light selected by the first diffraction region 251 serving as the inner
ring zone are as above, the diffraction orders (k1m, k2m) of light
selected by the second diffraction region 252 serving as the middle ring
zone and made dominant, and condensed onto the signal recording face of
the corresponding optical disc via the object lens 234, are (+1, +1),
(-1, -1), (0, +2), (0, -2), (0, +1), (0, -1), (+1, 0), or (-1, 0),
whereby a configuration can be realized in a staircase form or
non-cyclical form diffraction structure which is advantageous regarding
diffraction efficiency for example, whereby the functions of the inner
ring zone and middle ring zone can be each sufficiently manifested. That
is to say, the object lens 234 having the second diffraction region 252
configured thus is of a configuration wherein, at the time of configuring
the middle ring zone in particular, matching the image point position
with the diffraction functions at the inner ring zone and middle ring
zone such as described with respect to the second perspective is easier,
so optical beams of the first and second wavelengths input to the middle
ring zone can be placed in a state where the relation with the optical
beam of which aberration has been reduced as described above by the inner
ring zone is optimal, and also spherical aberration can be sufficiently
reduced. Further, with the object lens 234 having the second diffraction
region 252, high diffraction efficiency can be obtained regarding the
first and second wavelengths in a state of spherical aberration having
been corrected, and also suitable aperture restriction can be performed
regarding the third wavelength, and also the configuration is
advantageous from a manufacturing viewpoint. Accordingly, the object lens
234 having the diffraction unit 250 realizes condensing a suitable spot
on the signal recording face of the corresponding optical disc with high
light use efficiency, with a more advantageous configuration taking into
consideration advantages of configuration and so forth.

[0480]Further, with the object lens 234 having the diffraction unit 250
such as described above, when the diffraction orders (k1i, k2i, k3i) of
light selected by the first diffraction region 251 serving as the inner
ring zone are as above, the diffraction orders (k1m, k2m) of light
selected by the second diffraction region 252 serving as the middle ring
zone and made dominant, and condensed onto the signal recording face of
the corresponding optical disc via the object lens 234, are (+3, +2),
(-3, -2), (+2, +1), or (-2, -1), whereby a configuration can be realized
in a blazed form or non-cyclical form diffraction structure which is
advantageous regarding diffraction efficiency for example, whereby the
functions of the inner ring zone and middle ring zone can be each
sufficiently manifested. That is to say, the object lens 234 having the
second diffraction region 252 configured thus is of a configuration
wherein, at the time of configuring the middle ring zone in particular,
matching the image point position with the diffraction functions at the
inner ring zone and middle ring zone such as described with respect to
the second perspective is easier, so optical beams of the first and
second wavelengths input to the middle ring zone can be placed in a state
where the relation with the optical beam of which aberration has been
reduced by the inner ring zone as described above is optimal, and also
spherical aberration can be sufficiently reduced. Further, with the
object lens 234 having the second diffraction region 252, high
diffraction efficiency can be obtained regarding the first and second
wavelengths in a state of spherical aberration having been corrected, and
also suitable aperture restriction can be performed regarding the third
wavelength, and also the configuration is advantageous from a
manufacturing viewpoint. Accordingly, the object lens 234 having the
diffraction unit 250 realizes condensing a suitable spot on the signal
recording face of the corresponding optical disc with high light use
efficiency, with a more advantageous configuration taking into
consideration advantages of configuration and so forth.

[0481]Further, with the object lens 234 having the diffraction unit 250
such as described above, when the diffraction orders (k1i, k2i, k3i) of
light selected by the first diffraction region 251 serving as the inner
ring zone are as above, the diffraction orders (k1m, k2m) of light
selected by the second diffraction region 252 serving as the middle ring
zone and made dominant, and condensed onto the signal recording face of
the corresponding optical disc via the object lens 234, are (+1, -1), or
(-1, +1), whereby a configuration can be realized in a staircase form or
non-cyclical form diffraction structure which is advantageous regarding
diffraction efficiency for example, and also (k1m, k2m) are (+1, +1), or
(-1, -1), whereby a configuration can be realized in a blazed form or
non-cyclical form diffraction structure which is advantageous regarding
diffraction efficiency for example, whereby the functions of the inner
ring zone and middle ring zone can be each sufficiently manifested. That
is to say, the object lens 234 having the second diffraction region 252
configured thus is of a configuration wherein, due to being used along
with a configuration wherein the effects of unwanted light are reduced by
a technique such as setting the return power or the optical pickup
optical system higher, at the time of configuring the middle ring zone in
particular, matching the image point position with the diffraction
functions at the inner ring zone and middle ring zone such as described
with respect to the second perspective is easier, so optical beams of the
first and second wavelengths input to the middle ring zone can be placed
in a state where the relation with the optical beam of which aberration
has been reduced by the inner ring zone as described above is optimal,
and also spherical aberration can be sufficiently reduced. Further, with
the object lens 234 having the second diffraction region 252, high
diffraction efficiency can be obtained regarding the first and second
wavelengths in a state of spherical aberration having been corrected, and
also suitable aperture restriction can be performed regarding the third
wavelength, and also the configuration is advantageous from a
manufacturing viewpoint. Accordingly, the object lens 234 having the
diffraction unit 250 realizes condensing a suitable spot on the signal
recording face of the corresponding optical disc with high light use
efficiency, with a more advantageous configuration taking into
consideration advantages of configuration and so forth.

[0482]Also, the diffraction unit 250 having the first through third
diffraction regions 251, 252, and 253 is configured such that the optical
beam of the third wavelength passing through the second and third
diffraction regions 252 and 253 results in the diffracted light of a
diffraction order output with maximum diffraction efficiency and a
predetermined diffraction efficiency being flared and the imaging
position is shifted from the signal recording face, thereby reducing the
diffraction efficiency of the diffracted light of the diffraction order,
whereby, with regard to the optical beam of the third wavelength, only
the portion of the optical beam which has passed through the first
diffraction region 251 is condensed on the signal recording face of the
optical disc by the object lens 234, and the first diffraction region 251
is formed to a size such that the optical beam of the third wavelength
passing through this region is shaped to have a size of a predetermined
numerical aperture, whereby aperture restriction can be performed
regarding the optical beam of the third wavelength so as to have a
numerical aperture of around 0.45, for example.

[0483]Also, the diffraction unit 250 is configured such that that the
optical beam of the second wavelength passing through the third
diffraction regions 253 results in the diffracted light of a diffraction
order output with maximum diffraction efficiency and a predetermined
diffraction efficiency being flared and the imaging position is shifted
from the signal recording face, thereby reducing the diffraction
efficiency of the diffracted light of the diffraction order, whereby,
with regard to the optical beam of the second wavelength, only the
portion of the optical beam which has passed through the first and second
diffraction regions 251 and 252 is condensed on the signal recording face
of the optical disc by the object lens 234, and the first and second
diffraction regions 251 and 252 are formed to a size such that the
optical beam of the second wavelength passing through this region is
shaped to have a size of a predetermined numerical aperture, whereby
aperture restriction can be performed regarding the optical beam of the
second wavelength so as to have a numerical aperture of around 0.60, for
example.

[0484]Also, the diffraction unit 250 performs places the optical beam of
the first wavelength passing outside of the third diffraction region 253
in a state so as to not be suitably condensed on the signal recording
face of the corresponding type of optical disc via the object lens 234,
or shields the optical beam of the first wavelength passing outside of
the third diffraction region 253, whereby, with regard to the optical
beam of the first wavelength, only the optical beam portion which has
passed through the first through third diffraction regions 251, 252, and
253 is condensed on the signal recording face of the optical disc via the
object lens 234, and also, the first through third diffraction regions
251, 252, and 253 are formed to a size which is the predetermined
numerical aperture of the first wavelength optical beam passing through
this region, whereby aperture restriction can be performed regarding the
optical beam of the first wavelength such that NA= around 0.85, for
example.

[0485]Thus, the diffraction unit 250 provided on one face of the by the
object lens 234 disposed on the optical path as described above not only
realizes three-wavelength compatibility, but also enables optical beams
of each wavelength to be input to the common object lens 234 in a state
wherein aperture restriction is performed with a numerical aperture
appropriate for each of the three types of optical discs and optical
beams of the first through third wavelengths, thereby not only having
functions of aberration correction corresponding to the three
wavelengths, but also serving as an aperture restricting unit.

[0486]It should be noted that a diffraction unit can be configured by
suitably combining the diffraction regions in the above-described
embodiments. That is to say, the diffraction order of each wavelength
passing through each diffraction region can be selected as appropriate.
In the event of changing the diffraction order of each wavelength passing
through each diffraction region, an object lens 234 having a lens curve
face corresponding to each diffraction order of each wavelength passing
through each diffraction region can be used.

[0487]Also, while description has been made above with the diffraction
unit 250 configured of the three diffraction regions 251, 252, and 253
formed on the incident side face of the object lens 234, as shown in FIG.
58A, the present invention is not restricted to this arrangement, and may
be provided to the output side of the object lens 234. Further, the
diffraction unit 250 having the first through third diffraction regions
251, 252, and 253, can be integrally configured on the input or output
side of an optical element provided separately from the object lens 234,
and as shown in FIG. 58B for example, a condensing optical device may be
configured including an object lens 234B which has only a lens curve with
the diffraction unit 250 removed therefrom, and a diffraction optical
element 235B with the diffraction unit 250 provided on one face thereof
and disposed on the optical path shared by the three wavelengths. With
the object lens 234 shown in FIG. 58A for example, the planar shape of a
diffraction structure required for the functions of diffractive power is
combined with a reference face at the incident side required for the lens
to be able to have functions of refractive power, conversely, in the case
shown in FIG. 58B wherein a separate diffraction optical element 235B is
provided, the object lens 234B itself has the planar shape of a
diffraction structure required for the functions of refractive power, and
the diffraction optical element 235B has formed on one face thereof a
diffraction structure required for the functions of diffractive power.
The object lens 234B and diffraction optical element 235B shown in FIG.
58B function as a condensing optical device in the same way as the
above-described object lens 234, so as to reduce aberration and the like
and also realize three-wavelength compatibility of the optical pickup due
to being used as the optical pickup and manifests advantages of enabling
further reduction in optical parts and also simplification of
configuration and reduction in configuration size, high productivity, and
low costs, and the diffraction structure can be made more complex as
compared with a case of integrally providing on the object lens 234. On
the other hand, the arrangement shown in FIG. 58A described above
functioning as a condensing optical device which suitably condenses the
optical beams of three different wavelengths on the signal recording face
of respectively corresponding optical discs such that spherical
aberration does not occur, with the single element (object lens 234)
alone configured of the diffraction unit 250 integrally provided to the
object lens 234, enables further reduction in optical parts and reduction
in size of the configuration. Note that the above-described diffraction
unit 250 sufficiently manifests the advantages thereof with the
diffraction structure for aberration correction to realize
three-wavelength compatibility being provided on a single face that has
been difficult with the related art, which enables such a refractive
element to be integrally formed with the object lens 234, further
enabling directly forming a diffraction face on a plastic lens, and
forming the object lens 234 with which the diffraction unit 250 has been
integrated of a plastic material further realizes improved production and
lower costs.

[0488]The collimator lens 242 provided between the object lens 234 and the
third beam splitter 238 converts the divergent angle of each of the first
through third wavelength optical beams of which the optical paths have
been synthesized at the second beam splitter 237 and passed through the
third beam splitter 238, and outputs to the quarter-wave plate 243 and
object lens 234 side, in a generally parallel light state, for example.
The arrangement wherein the collimator lens 242 inputs the optical beams
of the first and second wavelengths into the above-described object lens
234 with the divergent angle thereof in the state of generally parallel
light, and also inputs the optical beam of the third wavelength into the
object lens 234 in a state which is slightly diffused or converged as to
parallel light (hereinafter also referred to as "finite system state")
enables further reduction of spherical aberration at the time of
condensing the second and third wavelength optical beams on the signal
recording face of the second and third optical discs via object lens 234,
realizing three-wavelength compatibility with even less aberration
occurring. This point has been described above with reference to FIGS. 41
and 42. While an arrangement has been described here wherein the optical
beam of the third wavelength is input to the object lens 234 in a state
of a predetermined divergent angle, due to the positional relation
between the second light source 232 having the second emitting unit for
emitting the second wavelength optical beam and the collimator lens 242,
and/or the positional relation between the third light source 233 having
the third emitting unit for emitting the third wavelength optical beam
and the collimator lens 242, in the event of positioning multiple
emitting units at a common light source for example, this may be realized
by providing an element which converts only the divergent angle of the
optical beam of the second and/or third wavelengths, or by inputting into
the object lens 234 in a predetermined divergent angle state by providing
a mechanism to drive the collimator lens 242 or the like. Also, either
the optical beams of the second wavelength, or the optical beams of the
second and third wavelengths, may be input to the object lens 234 in the
finite system state in accordance with the situation, thereby further
reducing aberration. Also, optical beams of the second and third
wavelengths may be input in the finite system state and in a diffused
state, thereby realizing adjustment of return power and even more
excellent optical system compatibility may be achieved by setting the
focus capture range and so forth to a desired state matching the format
by adjusting the return power.

[0489]The multi-lens 246 is, for example, a wavelength-selective
multi-lens, whereby the returning first through third wavelength optical
beams separated from the outgoing path optical beams by being reflected
at the third beam splitter 238, after having been reflected off of the
signal recording face of the respective optical disc, and passed through
the object lens 234, redirecting mirror 244, quarter-wave plate 243, and
collimator lens 242, is appropriately condensed on the photoreception
face of the photodetector or the like of the photosensor 245. At this
time, the multi-lens 246 provides the return optical beam with
astigmatism for detection of focus error signals or the like.

[0490]The photosensor 245 receives the return optical beam condensed at
the multi-lens 246, and detects, along with information signals, various
types of detection signals such as focus error signals, tracking error
signals, and so forth.

[0491]With the optical pickup 203 configured as described above, the
object lens 234 is driven so as to be displaced based on the focus error
signals and tracking error signals obtained by the photosensor 245,
whereby the object lens 234 is moved to a focal position as to the signal
recording face of the optical disc 2, the optical beam is focused onto
the signal recording face of the optical disc 2, and information is
recorded to or played from the optical disc 2.

[0492]The optical pickup 203 is provided on one face of the object lens
234, can provide optical beams of each wavelength with a diffraction
efficiency and diffraction angle suitable for each region due to the
diffraction unit 250 having the first through third diffraction regions
251, 252, and 253, can sufficiently reduce spherical aberration at the
signal recording face of the three types of first through third optical
discs 11, 12, and 13, of which the format for the thickness of the
protective layer or the like differs, and enables reading and writing of
signals to and from the multiple types of optical discs 11, 12, and 13,
using optical beams of three different wavelengths.

[0493]Also, the object lens 234 having the diffraction unit 250 shown in
FIG. 58A, and the diffraction optical element 235B having the diffraction
unit 250 and object lens 234B described with reference to FIG. 58B,
making up the above-described optical pickup 203, can each function as a
condensing optical device for condensing input optical beams at
predetermined positions. In the event of using this condensing optical
device for an optical pickup which performs recording and/or playing of
information signals by irradiating optical beams onto three different
types of optical discs, the diffraction unit 250 provided on one face of
the object lens 234 or the diffraction optical element 235B enables the
condensing optical device to appropriately condense corresponding optical
beams onto the signal recording face of the three types of optical discs
in a state with spherical aberration sufficiently reduced, meaning that
three-wavelength compatibility of the optical pickup using the object
lens 234 or the object lens 234B common to the three wavelengths can be
realized.

[0494]Also, while the diffraction optical element 235B having the
diffraction unit 250 and object lens 234B described with reference to
FIG. 58B for example may be provided to an actuator such as an object
lens driving mechanism or the like for driving the object lens 234B such
that the diffraction optical element 235B having the diffraction unit 250
and the object lens 234B are integral, this may be configured as a
condensing optical unit wherein the diffraction optical element 235B and
object lens 234B are formed as an integrated unit, in order to improve
precision of assembly to the lens holder of the actuator, and facilitate
assembly work. For example, a condensing optical unit can be configured
by using spacers or the like to fix the diffraction optical element 235B
and object lens 234B to the holder while setting the positioning,
spacing, and optical axis, so as to be integrally formed. Due to being
integrally assembled to the object lens driving mechanism as described
above, the diffraction optical element 235B and object lens 234B can
appropriately condense the first through third wavelength optical beams
on the signal recording face of the respective optical discs with
spherical aberration reduced, even at the time of field shift such as
displacement in the tracking direction, and so forth, for example.

[0495]Next, the optical paths of the optical beams emitted from the first
through third light sources 231, 232, and 233 of the optical pickup 203
configured as described above, will be described with reference to FIG.
37. First, the optical path at the time of emitting the optical beam of
the first wavelength as to the first optical disc 11 and performing
reading or writing of information will be described.

[0496]The disc type determination unit 22 which has determined that the
type of the optical disc 2 is the first optical disc 11 causes the
optical beam of the first wavelength to be emitted from the first
emitting unit of the first light source 231.

[0497]The optical beam of the first wavelength is split into three beams
by the first grating 239, for detection of tracking error signals and so
forth, and is input to the second beam splitter 237. The optical beam of
the first wavelength which has been input to the second beam splitter 237
is reflected at a mirror face 237a thereof, and is output to the third
beam splitter 238 side.

[0498]The optical beam of the first wavelength which is input to the third
beam splitter 238 is transmitted through a mirror face 238a thereof,
output to the collimator lens 242 side, where the divergent angle is
converted by the collimator lens 242 so as to be generally parallel
light, provided with a predetermined phase difference at the quarter-wave
plate 243, reflected off of the redirecting mirror 244, and output to the
object lens 234 side.

[0499]The optical beam of the first wavelength which is input to the
object lens 234 is diffracted with the optical beam which has passed
through each region thereof having a predetermined diffraction order
dominant therein as described above, due to the first through third
diffraction regions 251, 252, and 253 of the diffraction unit 250
provided on the incident side face thereof, and also suitably condensed
on the signal recording face of the first optical disc 11 due to the
refractive power of the lens curved face of the object lens 234. At this
time, the optical beam of the first wavelength is provided with
diffractive power such that the optical beam passing through the regions
251, 252, and 253 is in a state where spherical aberration can be
reduced, and accordingly can be suitably condensed. Note that the optical
beam of the first wavelength output from the object lens 234 is not only
in a state of a predetermined divergent angle, but also is in a state of
aperture restriction.

[0500]The optical beam condensed at the first optical disc 11 is reflected
at the signal recording face, passes through the object lens 234,
redirecting mirror 244, quarter-wave plate 243, and collimator lens 242,
is reflected off of the mirror face 238a of the third beam splitter 238,
and is output to the photosensor 245 side.

[0501]The optical beam split from the optical path of the outgoing optical
beam reflected off of the third beam splitter 238 is condensed on the
photoreception face of the photodetector 245 by the multi-lens 246, and
detected.

[0502]Next, description will be made regarding the optical path at the
time of emitting an optical beam of the second wavelength to the second
optical disc 12 and reading or writing information. The disc type
determination unit 22 which has determined that the type of the optical
disc 2 is the second optical disc 12 causes the optical beam of the
second wavelength to be emitted from the second emitting unit of the
second light source 232.

[0503]The optical beam of the second wavelength emitted form the second
emitting unit is split into three beams by the second grating 240, for
detection of tracking error signals and so forth, and is input to the
first beam splitter 236. The optical beam of the second wavelength which
has been input to the first beam splitter 236 is transmitted through a
mirror face 236a thereof, also transmitted through the mirror face 237a
of the second beam splitter 237, and is output to the third beam splitter
238 side.

[0504]The optical beam of the second wavelength which is input to the
third beam splitter 238 is transmitted through the mirror face 238a
thereof, output to the collimator lens 242 side, where the divergent
angle is converted by the collimator lens 242 so as to be in a state of
diffused light, provided with a predetermined phase difference at the
quarter-wave plate 243, reflected off of the redirecting mirror 244, and
output to the object lens 234 side.

[0505]The optical beam of the second wavelength which is input to the
object lens 234 is diffused with the optical beam which has passed
through each region thereof having a predetermined diffraction order
dominant therein as described above, due to the first and second
diffraction regions 251 and 252 of the diffraction unit 250 provided on
the incident side face thereof, and also suitably condensed on the signal
recording face of the second optical disc 12 due to the refractive power
of the lens curved face of the object lens 234. At this time, the optical
beam of the second wavelength is provided with diffractive power such
that the optical beam passing through the first and second diffraction
regions 251 and 252 is in a state where spherical aberration can be
reduced, and accordingly can be suitably condensed. Also note that the
diffracted light due to the optical beam of the second wavelength which
having passed through the third diffraction region 253 is in a state of
not being condensed on the signal recording face of the second optical
disc, i.e., suitable aperture restriction advantages can be had, due to
the advantages of the above-described flaring.

[0506]The return optical path of the optical beam reflected off of the
signal recording face of the second optical disc 12 is the same as with
the case of the above-described optical beam of the first wavelength, and
accordingly description thereof will be omitted.

[0507]Next, description will be made regarding the optical path at the
time of emitting an optical beam of the third wavelength to the third
optical disc 13 and reading or writing information. The disc type
determination unit 22 which has determined that the type of the optical
disc 2 is the third optical disc 13 causes the optical beam of the third
wavelength to be emitted from the third emitting unit of the third light
source 233.

[0508]The optical beam of the third wavelength emitted form the third
emitting unit is split into three beams by the third grating 241, for
detection of tracking error signals and so forth, and is input to the
first beam splitter 236. The optical beam of the third wavelength which
has been input to the first beam splitter 236 is reflected off of the
mirror face 236a thereof, transmitted through the mirror face 237a of the
second beam splitter 237, and is output to the third beam splitter 238
side.

[0509]The optical beam of the third wavelength which is input to the third
beam splitter 238 is transmitted through the mirror face 238a thereof,
output to the collimator lens 242 side, where the divergent angle is
converted by the collimator lens 242 so as to be in a diffused light
state, provided with a predetermined phase difference at the quarter-wave
plate 243, reflected off of the redirecting mirror 244, and output to the
object lens 234 side.

[0510]The optical beam of the third wavelength which is input to the
object lens 234 is diffused with the optical beam which has passed
through each region thereof having a predetermined diffraction order
dominant therein as described above, due to the first diffraction region
251 of the diffraction unit 250 provided on the incident side face
thereof, and also suitably condensed on the signal recording face of the
third optical disc 13 due to the refractive power of the lens curved face
of the object lens 234. At this time, the optical beam of the third
wavelength is provided with diffractive power such that the optical beam
passing through the first diffraction region 251 is in a state where
spherical aberration can be reduced, and accordingly can be suitably
condensed. Also note that the diffracted light due to the optical beam of
the third wavelength which having passed through the second and third
diffraction regions 252 and 253 is in a state of not being condensed on
the signal recording face of the third optical disc 13, i.e., suitable
aperture restriction advantages can be had, due to the advantages of the
above-described flaring.

[0511]The return optical path of the optical beam reflected off of the
signal recording face of the third optical disc 13 is the same as with
the case of the above-described optical beam of the first wavelength, and
accordingly description thereof will be omitted.

[0512]Note that while a configuration has been described here wherein the
optical beam of the second and third wavelengths have the position of the
second and/or third emitting units adjusted such that the optical beam of
which the divergent angle is converted by the collimator lens 242 and
input to the object lens 234 is in a diffused state as to generally
parallel light, a configuration may be made wherein the optical beam is
input to the object lens 234 by providing an element which has wavelength
selectivity and converts the divergent angle, or by providing a mechanism
which drives the collimator lens 242 in the optical axis direction in a
diffused or converged state.

[0513]Also, while description has been made regarding a configuration
wherein the optical beam of the first wavelength is input to the object
lens 234 in a state of generally parallel light, the optical beams of the
second and third wavelengths are input to the object lens 234 in a state
of diffused light, the present invention is not restricted to this
arrangement, and configurations may be made wherein, for example, the
first through third wavelength optical beams are selectively input to the
object lens 234 in a state of diffused light, parallel light, or
converged light.

[0514]The optical pickup 203 to which the present invention has been
applied has first through third emitting units for emitting optical beams
of first through third wavelengths, an object lens 234 for condensing the
optical beams of first through third wavelengths emitted from the first
through third emitting units into a signal recording face of an optical
disc, and a diffraction unit 250 provided on one face of the object lens
234 serving as an optical element disposed on the outgoing optical path
of the optical beams of first through third wavelengths, wherein the
diffraction unit 250 has first through third diffraction regions 251,
252, and 253, with the first through third diffraction regions 251, 252,
and 253 being different diffraction structures ring shaped and having a
predetermined depth, and the first through third diffraction structures
whereby optical beams of each wavelength are diffracted such that
diffracted light of a predetermined diffraction order is dominant as
described above, and according to this configuration, optical beams
corresponding to each of three types of optical discs having difference
usage wavelengths can be appropriately condensed on the signal recording
face using a common object lens 234, thereby realizing excellent
recording and/or playing of information signals to/from the respective
optical discs by realizing three-wavelength compatibility with the common
object lens 234, without necessitating a complex structure.

[0515]That is to say, the optical pickup 203 to which the present
invention has been applied obtains optimal diffraction efficiencies and
diffraction angels for the first through third wavelength optical beams
due to the diffraction unit 250 provided on one face within the optical
path thereof, whereby signals can be read from and written to the
multiple types of optical discs 11, 12, and 13, using the optical beams
of different wavelengths emitted from the multiple emitting units
provided to each of the light sources 231, 232, and 233, and also optical
parts such as the object lens 234 and so forth can be shared, thereby
reducing the number of parts, simplifying and reducing the size of the
configuration, and realizing high production and lower costs.

[0516]Also, the optical pickup 203 to which the present invention has been
applied is configured having the relation k1i≧k2i>k3i for the
predetermined diffraction orders (k1i, k2i, k3i) selected by the first
diffraction region 251 serving as the inner ring zone, so condensing
diffracted light in a state in which spherical aberration can be reduced
on the signal recording face of the corresponding optical discs maximizes
diffraction efficiency, which is to say in the case of using the third
wavelength λ3, the focal distance can be prevented from becoming
too long as to the first wavelength λ1 in order to ensure operating
distance thereof, thereby preventing problems such as the lens diameter
of the object lens being large, the overall size of the optical pickup
being large, and so forth. Reducing the lens diameter of the object lens
facilitates design of the actuator, and the focal distance can be
shortened, thereby obtaining excellent aberration properties.
Accordingly, information signals can be suitably recorded to and/or
played from respective optical discs with excellent compatibility being
realized, the configuration can be further simplified and the size
reduced, realizing high productivity and low costs.

[0517]Also, the optical pickup 203 to which the present invention has been
applied is configured such that, of the diffraction orders (k1i, k2i,
k3i) selected by the first diffraction region 251 serving as the inner
ring zone, k1i and k3i are (-2, -3), (-1, -2), (-1, -3), (0, -2), (0,
-3), (1, -2), (1, -3), (2, -1), (2, -2), (2, -3), (3, 0), (3, -1), (3,
-2), or (3, -3), thereby preventing problems such as the lens diameter of
the object lens being large, the overall size of the device being large,
and so forth, with the operating distance and focal distance for each
wavelength being in a suitable state, and additionally, the grooves are
prevented from becoming too deep, whereby the manufacturing process can
be simplified, and also deterioration of forming precision can be
prevented. Accordingly, information signals can be suitably recorded to
and/or played from respective optical discs with excellent compatibility
being realized, the configuration can be simplified and the size reduced
while facilitating manufacturing, realizing high productivity and low
costs.

[0518]Also, the optical pickup 203 to which the present invention has been
applied is configured such that the first diffraction region 251 serving
as the inner ring zone which provides the three wavelengths with
predetermined diffractive power and needs high diffraction efficiency,
has formed a stepped diffraction structure, thereby suppressing the
amount of diffracted light of unwanted light, preventing deterioration of
jittering and the like due to unwanted light being received at the
photosensor, and also, even in cases of a certain amount of diffracted
light of unwanted light occurring, unwanted light being received at the
photosensor at the time of focusing leading to deterioration of jittering
and the like can be prevented by making the diffraction order of the
unwanted light to be a deviated order with great diffraction angle
difference, that is other than an adjacent diffraction order of the focus
light.

[0519]Also, the optical pickup 203 to which the present invention has been
applied is configured having the outer ring zone formed integrally on one
face of the object lens 234 and also provided on the outermost side
thereof, formed as a blazed form diffraction structure at the third
diffraction region 235, which is an advantageous structure in the case of
forming a diffraction structure at portions having an extremely steep
lens curved surface, such as with a three-wavelength-compatible lens,
whereby manufacturing can be facilitated and deterioration in forming
precision can be prevented.

[0520]Also, the optical pickup 203 to which the present invention has been
applied is configured such that the diffraction orders (k1i, k2i, k3i) of
light selected by the first diffraction region 251 are (1, -1, -2), (0,
-1, -2), (1, -2, -3) or (0, -2, -3), and the diffraction structure is
configured in a staircase form, so adverse affects of unwanted light can
be suppressed, the operating distance and focal distance for each
wavelength can be made to be in a suitable state and the lens diameter of
the object lens and the size of the device can be prevented from being
large, and additionally, the grooves are prevented from becoming too
deep, whereby the manufacturing process can be simplified, and also
deterioration of forming precision can be prevented. Accordingly,
information signals can be suitably recorded to and/or played from
respective optical discs with excellent compatibility being realized, the
configuration can be simplified and the size reduced while facilitating
manufacturing, realizing high productivity and low costs.

[0521]Also, with the optical pickup 203 to which the present invention has
been applied, in addition to the diffraction order selected by the inner
ring zone, the diffraction orders (k1m, k2m) of light selected by the
second diffraction region 252 serving as the middle ring zone are (+1,
+1), (-1, -1), (0, +2), (0, -2), (0, +1), (0, -1), (+1, 0), (-1, 0), (+1,
-1), or (-1, +1), and the diffraction structure is configured as a
staircase form or non-cyclical form, whereby the functions of the inner
ring zone and middle ring zone can be each sufficiently manifested.
Accordingly, information signals can be suitably recorded to and/or
played from respective optical discs with excellent compatibility being
realized, the configuration can be simplified and the size reduced while
facilitating manufacturing, realizing high productivity and low costs.

[0522]Also, with the optical pickup 203 to which the present invention has
been applied, in addition to the diffraction order selected by the inner
ring zone, the diffraction orders (k1m, k2m) of light selected by the
second diffraction region 252 serving as the middle ring zone are (+3,
+2), (-3, -2), (+2, +1), (-2, -1), (+1, -1), or (-1, -1), and the
diffraction structure is configured as a blazed form or non-cyclical
form, whereby the functions of the inner ring zone and middle ring zone
can be each sufficiently manifested. Accordingly, information signals can
be suitably recorded to and/or played from respective optical discs with
excellent compatibility being realized, the configuration can be
simplified and the size reduced while facilitating manufacturing,
realizing high productivity and low costs.

[0523]Also, with the optical pickup 203 to which the present invention has
been applied, at the time of input of the condensing optical device such
as the object lens 234 or the like, the optical beam of the first
wavelength is generally parallel light and the optical beams of the
second and third wavelengths are input as diffused light, and due to this
configuration, the optical beams passing through the first diffraction
region 251 serving as the inner ring zone can be suitably condensed on
the signal recording face of the corresponding optical disc in a state of
high diffractive efficiency and even further reduced spherical
aberration, and also the advantages of flaring can be had at the second
and third diffraction regions serving as the middle ring zone and outer
ring zone, high efficiency and reduced spherical aberration can be
realized for optical beams of a predetermined wavelength while the
quantity of light input to the corresponding signal recording face can be
reduced for optical beams of wavelengths regarding which condensing is
undesirable, and further, the freedom of diffraction order selection can
be improved and simplification of configuration and so forth realized.

[0524]Also, the optical pickup 203 to which the present invention has been
applied can share the object lens 234 between the three wavelengths,
thereby preventing trouble such as reduction of sensitivity of the
actuator due to increased weight of moving parts, and the attachment
angle of the actuator to lens holder being unsuitable, and so forth.
Also, the optical pickup 203 to which the present invention has been
applied can sufficiently reduce spherical aberration which is problematic
in the case of sharing the object lens 234 between the three wavelengths,
due to the diffraction unit 250 provided on one face of the optical
element (object lens 234, diffraction optical element 235B), so problems
such as positioning of diffraction units in the event that diffraction
units are provided on multiple faces to reduce spherical aberration as
with the related art, and deterioration of diffraction efficiency due to
providing of the multiple diffraction units and so forth, can be
prevented, which realizes simplification of the assembly process and
improved usage efficiency of light. Also, with the optical pickup 203 to
which the present invention has been applied, a configuration such as
described above, wherein the diffraction unit 250 is provided on one face
of the optical element enables a configuration having the object lens 234
integrally formed with the diffraction unit 250, realizes further
simplification of the configuration, reduction in weight of moving parts
of the actuator, simplification of the assembly process, and improved
usage efficiency of light.

[0525]Further, as shown in FIGS. 58A and 58B described above, with the
optical pickup 203 to which the present invention has been applied, the
diffraction unit 250 provided on one face of the object lens 234 or
diffraction optical element 235B not only realizes three-wavelength
compatibility, but also enables aperture restriction by numerical
aperture to be performed corresponding to the three types of optical
discs and optical beams of the three wavelengths, thereby doing away with
the need for aperture restriction filters or the like which have been
necessary with the related art, and also adjustment in the positioning
thereof, realizing further simplification of the configuration, reduction
in size, and reduction in costs.

[0526]Also, the optical pickup 203 has been described above with a
configuration wherein the first emitting unit is provided at the first
light source 231, the second emitting unit is provided at the second
light source 232, and the third emitting unit is provided at the third
light source 233, the invention is not restricted to this, and an
arrangement may be made wherein two emitting units of the first through
third emitting units are disposed at one light source and the remaining
emitting unit is disposed at another light source, for example.

[0527]Next, description will be made regarding an optical pickup 260 shown
in FIG. 59 including a light source having a first emitting unit, and a
light source having second and third emitting units. Note that portions
in the following description which are the same as with the optical
pickup 203 will be denoted with the same reference numerals, and
description thereof will be omitted.

[0528]As shown in FIG. 59, the optical pickup 260 to which the present
invention has been applied includes a first light source 261 having a
first emitting unit for emitting an optical beam of a first wavelength, a
second light source 262 having a second emitting unit for emitting an
optical beam of a second wavelength and a third emitting unit for
emitting an optical beam of a third wavelength, and an object lens 234
serving as a condensing optical device for condensing optical beams
emitted from the first through third emitting units onto the signal
recording face of an optical disc 2. Also, with the optical pickup 260
described here as well, a configuration may be made wherein a condensing
optical device configured of the object lens 234B and the diffraction
optical element 235B having the diffraction unit 250 such as shown in
FIG. 58B is provided instead of the object lens 234 having the
diffraction unit 250 described here.

[0529]Also, the optical pickup 260 includes a beam splitter 263 serving as
an optical path synthesizing unit for synthesizing the optical paths of
the optical beam of the first wavelength that has been emitted from the
first emitting unit of the first light source 261 and the optical beams
of the second and third wavelengths that have been emitted from the
second and third emitting units of the second light source 262, and a
beam splitter 264 serving the same function as the above third beam
splitter 238.

[0530]Further, the optical pickup 260 has a first grating 239, and a
grating 265 with wavelength dependency, provided between the second light
source unit 262 and the beam splitter 263, for diffracting the optical
beams of the second and third wavelengths that have been emitted from the
second and third emitting units into three beams, for detection of
tracking error signals and so forth.

[0531]Also, the optical pickup 260 has a collimator lens 242, quarter-wave
plate 243, redirecting mirror 244, photosensor 245, and multi-lens 246,
and also a collimator lens driving unit 266 for driving the collimator
lens 242 in the direction of the optical axis. The collimator lens
driving unit 266 can adjust the divergent angle of optical beams passing
through the collimator lens 242 as described above by driving the
collimator lens 242 in the direction of the optical axis, whereby not
only can spherical aberration be reduced by inputting each optical beam
to the object lens 234 in a predetermined state enabling the
above-described flaring, but in the event that the mounted optical disc
is a so-called multi-layer optical disc having multiple signal recording
faces, recording and/or playing to/from each of the signal recording
faces is enabled.

[0532]With the optical pickup 260 configured as described above, the
functions of each of the optical parts is the same as with the optical
pickup 203 except for those mentioned above, and the optical paths of the
optical beams of the first through third wavelengths emitted from the
first through third emitting units are the same as with the optical
pickup 203 except for the above-mentioned, i.e., following synthesizing
of the optical paths of the optical beams of each wavelength by the beam
splitter 264, so detailed description thereof will be omitted.

[0533]The optical pickup 260 to which the present invention has been
applied has first through third emitting units for emitting optical beams
of first through third wavelengths, an object lens 234 for condensing the
optical beams of first through third wavelengths emitted from the first
through third emitting units into a signal recording face of an optical
disc, and a diffraction unit 250 provided on one face of the object lens
234 serving as an optical element disposed on the outgoing optical path
of the optical beams of first through third wavelengths, wherein the
diffraction unit 250 has first through third diffraction regions 251,
252, and 253, with the first through third diffraction regions 251, 252,
and 253 being different diffraction structures ring shaped and having a
predetermined depth, and the first through third diffraction structures
whereby optical beams of each wavelength are diffracted such that
diffracted light of a predetermined diffraction order is dominant as
described above, and according to this configuration, optical beams
corresponding to each of three types of optical discs having different
usage wavelengths can be appropriately condensed on the signal recording
face using the single shared object lens 234, thereby realizing excellent
recording and/or playing of information signals to/from the respective
optical discs by realizing three-wavelength compatibility with the common
object lens 234, without necessitating a complex structure. The optical
pickup 260 also has the other advantages of the above-described optical
pickup 203, as well.

[0534]Further, the optical pickup 260 is configured such that the second
and third emitting units are positioned at a common light source 262,
thereby realizing further simplification of configuration and reduction
in size. Note that in the same way, with the optical pickup to which the
present invention has been applied, the first through third emitting
units may be positioned at a light source at generally the same position,
thereby realizing further simplification of configuration and reduction
in size with such a configuration.

[0535]The optical disc device 1 to which the present invention has been
applied has a driving unit for holding and rotationally driving an
optical disc arbitrarily selected from the first through third optical
discs, and an optical pickup for performing recording and/or playing of
information signals from/to the optical disc being rotationally driven by
the driving unit by selectively irradiating one of multiple optical beams
of different wavelengths corresponding to the optical disc, and by using
the above-described optical pickups 203 or 260 as the optical pickup,
optical beams corresponding to each of three types of optical discs
having different usage wavelengths can be appropriately condensed on the
signal recording face due to the diffraction unit provided on one face of
the optical element on the optical path of the optical beams of the first
through third wavelengths, using a single shared object lens 234, thereby
realizing excellent recording and/or playing properties by realizing
three-wavelength compatibility with the common object lens 234, while
enabling simplification of the structure and reduction in size, without
necessitating a complex structure.

[0536]It should be understood by those skilled in the art that various
modifications, combinations, sub-combinations and alterations may occur
depending on design requirements and other factors insofar as they are
within the scope of the appended claims or the equivalents thereof.